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Overview.html
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<!DOCTYPE html><html lang="en-US"><meta charset="utf-8">
<title>Fetch Standard</title>
<link href="https://resources.whatwg.org/standard.css" rel="stylesheet">
<link href="https://resources.whatwg.org/logo-fetch.svg" rel="icon">
<div class="head">
<p><a class="logo" href="https://whatwg.org/"><img alt="WHATWG" height="100" src="https://resources.whatwg.org/logo-fetch.svg" width="100"></a>
<h1 id="cors">Fetch</h1>
<h2 class="no-num no-toc" id="living-standard-—-last-updated-12-april-2016">Living Standard — Last Updated 12 April 2016</h2>
<dl>
<dt>Participate:
<dd><a href="https://github.com/whatwg/fetch">GitHub whatwg/fetch</a>
(<a href="https://github.com/whatwg/fetch/issues/new">file an issue</a>,
<a href="https://github.com/whatwg/fetch/issues">open issues</a>,
<a href="https://www.w3.org/Bugs/Public/buglist.cgi?product=WHATWG&component=Fetch&resolution=---">legacy open bugs</a>)
<dd><a href="https://wiki.whatwg.org/wiki/IRC">IRC: #whatwg on Freenode</a>
<dt>Commits:
<dd><a href="https://github.com/whatwg/fetch/commits">GitHub whatwg/fetch/commits</a>
<dd><a href="https://twitter.com/fetchstandard">@fetchstandard</a>
</dl>
<script async="" src="https://resources.whatwg.org/file-issue.js"></script>
<script defer="" id="head" src="https://resources.whatwg.org/dfn.js"></script>
</div>
<h2 class="no-num no-toc short" id="abstract">Abstract</h2>
<p>The Fetch standard defines requests, responses, and the process that binds them:
fetching.
<h2 class="no-num no-toc" id="table-of-contents">Table of Contents</h2>
<!--begin-toc-->
<ol class="toc">
<li><a class="no-num short" href="#goals">Goals</a></li>
<li><a class="short" href="#preface"><span class="secno">1 </span>Preface</a></li>
<li><a class="short" href="#conformance"><span class="secno">2 </span>Conformance</a></li>
<li><a href="#infrastructure"><span class="secno">3 </span>Infrastructure</a>
<ol>
<li><a href="#http"><span class="secno">3.1 </span>HTTP</a>
<ol>
<li><a href="#methods"><span class="secno">3.1.1 </span>Methods</a></li>
<li><a href="#terminology-headers"><span class="secno">3.1.2 </span>Headers</a></li>
<li><a href="#statuses"><span class="secno">3.1.3 </span>Statuses</a></li>
<li><a href="#bodies"><span class="secno">3.1.4 </span>Bodies</a></li>
<li><a href="#requests"><span class="secno">3.1.5 </span>Requests</a></li>
<li><a href="#responses"><span class="secno">3.1.6 </span>Responses</a></ol></li>
<li><a href="#authentication-entries"><span class="secno">3.2 </span>Authentication entries</a></li>
<li><a href="#fetch-groups"><span class="secno">3.3 </span>Fetch groups</a></li>
<li><a href="#connections"><span class="secno">3.4 </span>Connections</a></li>
<li><a href="#port-blocking"><span class="secno">3.5 </span>Port blocking</a></li>
<li><a href="#should-response-to-request-be-blocked-due-to-mime-type?"><span class="secno">3.6 </span>Should
<var>response</var> to <var>request</var> be blocked due to its MIME type?</a></li>
<li><a href="#referrer-policies"><span class="secno">3.7 </span>Referrer policies</a></li>
<li><a href="#client-hints-list"><span class="secno">3.8 </span>Client hints list</a></li>
<li><a href="#streams"><span class="secno">3.9 </span>Streams</a>
<ol>
<li><a href="#readablestream"><span class="secno">3.9.1 </span>ReadableStream</a></ol></ol></li>
<li><a href="#http-extensions"><span class="secno">4 </span>HTTP extensions</a>
<ol>
<li><a href="#origin-header"><span class="secno">4.1 </span>`<code title="">Origin</code>` header</a></li>
<li><a href="#http-cors-protocol"><span class="secno">4.2 </span>CORS protocol</a>
<ol>
<li><a href="#general"><span class="secno">4.2.1 </span>General</a></li>
<li><a href="#http-requests"><span class="secno">4.2.2 </span>HTTP requests</a></li>
<li><a href="#http-responses"><span class="secno">4.2.3 </span>HTTP responses</a></li>
<li><a href="#http-new-header-syntax"><span class="secno">4.2.4 </span>HTTP new-header syntax</a></ol></li>
<li><a href="#x-content-type-options-header"><span class="secno">4.3 </span>`<code title="">X-Content-Type-Options</code>` header</a>
<ol>
<li><a href="#should-response-to-request-be-blocked-due-to-nosniff?"><span class="secno">4.3.1 </span>Should
<var>response</var> to <var>request</var> be blocked due to nosniff?</a></ol></ol></li>
<li><a href="#fetching"><span class="secno">5 </span>Fetching</a>
<ol>
<li><a href="#main-fetch"><span class="secno">5.1 </span>Main fetch</a></li>
<li><a href="#basic-fetch"><span class="secno">5.2 </span>Basic fetch</a></li>
<li><a href="#http-fetch"><span class="secno">5.3 </span>HTTP fetch</a></li>
<li><a href="#http-redirect-fetch"><span class="secno">5.4 </span>HTTP-redirect fetch</a></li>
<li><a href="#http-network-or-cache-fetch"><span class="secno">5.5 </span>HTTP-network-or-cache fetch</a></li>
<li><a href="#http-network-fetch"><span class="secno">5.6 </span>HTTP-network fetch</a></li>
<li><a href="#cors-preflight-fetch"><span class="secno">5.7 </span>CORS-preflight fetch</a></li>
<li><a href="#cors-preflight-cache"><span class="secno">5.8 </span>CORS-preflight cache</a></li>
<li><a href="#cors-check"><span class="secno">5.9 </span>CORS check</a></ol></li>
<li><a href="#fetch-api"><span class="secno">6 </span>Fetch API</a>
<ol>
<li><a href="#headers-class"><span class="secno">6.1 </span>Headers class</a></li>
<li><a href="#body-mixin"><span class="secno">6.2 </span>Body mixin</a></li>
<li><a href="#request-class"><span class="secno">6.3 </span>Request class</a></li>
<li><a href="#response-class"><span class="secno">6.4 </span>Response class</a></li>
<li><a href="#structured-cloning-of-headers,-request,-and-response-objects"><span class="secno">6.5 </span>Structured cloning of <code>Headers</code>, <code>Request</code>, and <code>Response</code> objects</a></li>
<li><a href="#fetch-method"><span class="secno">6.6 </span>Fetch method</a></li>
<li><a href="#garbage-collection"><span class="secno">6.7 </span>Garbage collection</a></ol></li>
<li><a href="#websocket-protocol"><span class="secno">7 </span>WebSocket protocol alterations</a>
<ol>
<li><a href="#websocket-connections"><span class="secno">7.1 </span>Connections</a></li>
<li><a href="#websocket-opening-handshake"><span class="secno">7.2 </span>Opening handshake</a></ol></li>
<li><a class="no-num" href="#background-reading">Background reading</a>
<ol>
<li><a class="no-num" href="#http-header-layer-division">HTTP header layer division</a></li>
<li><a class="no-num" href="#atomic-http-redirect-handling">Atomic HTTP redirect handling</a></li>
<li><a class="no-num" href="#basic-safe-cors-protocol-setup">Basic safe CORS protocol setup</a></li>
<li><a class="no-num" href="#cors-protocol-and-http-caches">CORS protocol and HTTP caches</a></ol></li>
<li><a class="no-num" href="#references">References</a></li>
<li><a class="no-num" href="#acknowledgments">Acknowledgments</a></ol>
<!--end-toc-->
<h2 class="no-num short" id="goals">Goals</h2>
<p>To unify fetching across the web platform this specification supplants a number of algorithms and specifications:
<ul class="brief no-backref">
<li>HTML Standard's fetch and potentially CORS-enabled fetch algorithms
<a class="informative" href="#refsHTML">[HTML]</a>
<li>CORS <a class="informative" href="#refsCORS">[CORS]</a>
<li>HTTP `<a href="#http-origin"><code title="http-origin">Origin</code></a>` header semantics
<a class="informative" href="#refsORIGIN">[ORIGIN]</a>
</ul>
<p>Unifying fetching provides consistent handling of:
<ul class="brief">
<li>URL schemes
<li>Redirects
<li>Cross-origin semantics
<li>CSP <a href="#refsCSP">[CSP]</a>
<li>Service workers <a href="#refsSW">[SW]</a>
<li>Mixed Content <a href="#refsMIX">[MIX]</a>
<li>`<code title="">Referer</code>` <a href="#refsREFERRER">[REFERRER]</a>
</ul>
<h2 class="short" id="preface"><span class="secno">1 </span>Preface</h2>
<p>At a high level, fetching a resource is a fairly simple operation. A request goes in, a
response comes out. <!--You can't explain that! -->The details of that operation are
however quite involved and used to not be written down carefully and differ from one API
to the next.
<p>Numerous APIs provide the ability to fetch a resource, e.g. HTML's <code>img</code> and
<code>script</code> element, CSS' <code>cursor</code> and <code>list-style-image</code>,
the <code>navigator.sendBeacon()</code> and <code>self.importScripts()</code> JavaScript
APIs. The Fetch Standard provides a unified architecture for these features so they are
all consistent when it comes to various aspects of fetching, such as redirects and the
CORS protocol.
<p>The Fetch Standard also defines the <a href="#dom-global-fetch"><code title="dom-global-fetch">fetch()</code></a>
JavaScript API, which exposes most of the networking functionality at a fairly low level
of abstraction.
<h2 class="short" id="conformance"><span class="secno">2 </span>Conformance</h2>
<p>All diagrams, examples, and notes in this specification are
non-normative, as are all sections explicitly marked non-normative.
Everything else in this specification is normative.
<p>The key words "MUST", "MUST NOT", "REQUIRED", <!--"SHALL", "SHALL
NOT",--> "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" in the normative parts of this specification are to be
interpreted as described in RFC2119. For readability, these words do
not appear in all uppercase letters in this specification.
<a href="#refsRFC2119">[RFC2119]</a>
<h2 id="infrastructure"><span class="secno">3 </span>Infrastructure</h2>
<p>This specification uses terminology from the ABNF, Encoding, HTML, IDL, Streams, and URL Standards.
<a href="#refsABNF">[ABNF]</a>
<a href="#refsENCODING">[ENCODING]</a>
<a href="#refsHTML">[HTML]</a>
<a href="#refsWEBIDL">[WEBIDL]</a>
<a href="#refsSTREAMS">[STREAMS]</a>
<a href="#refsURL">[URL]</a>
<p>A <a class="external" data-anolis-spec="encoding" href="https://encoding.spec.whatwg.org/#byte">byte</a> sequence with bytes in the range 0x00 to
0x7F, inclusive, is represented as a <a class="external" data-anolis-spec="encoding" href="https://encoding.spec.whatwg.org/#utf-8">utf-8</a>-encoded
<a class="external" data-anolis-spec="encoding" href="https://encoding.spec.whatwg.org/#string">string</a> with code points in the range U+0000 to
U+007F, inclusive. To avoid confusion with an actual string, backticks are used.
<p class="example">"<code title="">true</code>" is a
<a class="external" data-anolis-spec="encoding" href="https://encoding.spec.whatwg.org/#string">string</a>, while `<code title="">true</code>` is a
byte sequence.
<p>Comparing two byte sequences in a <dfn id="byte-case-insensitive">byte-case-insensitive</dfn> manner means
comparing them exactly, byte for byte, except that the bytes in the range 0x41 to 0x5A,
inclusive, are considered to also match their corresponding byte in the range 0x61 to
0x7A, inclusive.
<p>A <dfn id="case-insensitive-byte-sequence">case-insensitive byte sequence</dfn> is a byte sequence that when compared to
another byte sequence does so in a <a href="#byte-case-insensitive">byte-case-insensitive</a> manner.
<p class="example">The
<a href="#case-insensitive-byte-sequence" title="case-insensitive byte sequence">case-insensitive byte sequences</a>
`<code title="">Content-Type</code>` and `<code title="">content-TYPE</code>` are equal.
<p>To <dfn id="byte-lowercase">byte-lowercase</dfn> a byte sequence, increase each byte it contains, in the
range 0x41 to 0x5A, inclusive, by 0x20.
<p>To <dfn id="byte-uppercase">byte-uppercase</dfn> a byte sequence, subtract each byte it contains, in the
range 0x61 to 0x7A, inclusive, by 0x20.
<hr>
<p>A <dfn id="fetch-url">fetch URL</dfn> is a <a class="external" data-anolis-spec="url" href="https://url.spec.whatwg.org/#concept-url" title="concept-url">URL</a> for
which implementations need not store the
<a class="external" data-anolis-spec="url" href="https://url.spec.whatwg.org/#concept-url-fragment" title="concept-url-fragment">fragment</a> as it is never
exposed. When <a class="external" data-anolis-spec="url" href="https://url.spec.whatwg.org/#concept-url-serializer" title="concept-url-serializer">serialized</a>,
the <i title="">exclude fragment flag</i> is set, meaning implementations can store the
<a class="external" data-anolis-spec="url" href="https://url.spec.whatwg.org/#concept-url-fragment" title="concept-url-fragment">fragment</a> nonetheless.
<hr>
<p><a class="external" data-anolis-spec="html" href="https://html.spec.whatwg.org/multipage/webappapis.html#concept-task" title="concept-task">Tasks</a> that are
<a class="external" data-anolis-spec="html" href="https://html.spec.whatwg.org/multipage/webappapis.html#queue-a-task" title="queue a task">queued</a> by this standard are annotated as one
of:
<ul class="brief">
<li><dfn id="process-request-body">process request body</dfn>
<li><dfn id="process-request-end-of-file">process request end-of-file</dfn>
<li><dfn id="process-response">process response</dfn>
<li><dfn id="process-response-end-of-file">process response end-of-file</dfn>
</ul>
<p>To <dfn id="queue-a-fetch-task">queue a fetch task</dfn> on <a href="#concept-request" title="concept-request">request</a>
<var>request</var> to <var>run an operation</var>, run these steps:
<ol>
<li><p>If <var>request</var>'s <a href="#concept-request-client" title="concept-request-client">client</a> is
null, terminate these steps.
<li><p><a class="external" data-anolis-spec="html" href="https://html.spec.whatwg.org/multipage/webappapis.html#queue-a-task">Queue a task</a> to
<var>run an operation</var> on <var>request</var>'s
<a href="#concept-request-client" title="concept-request-client">client</a>'s
<a class="external" data-anolis-spec="html" href="https://html.spec.whatwg.org/multipage/webappapis.html#responsible-event-loop">responsible event loop</a> using the
<a class="external" data-anolis-spec="html" href="https://html.spec.whatwg.org/multipage/webappapis.html#networking-task-source">networking task source</a>.
</ol>
<p>To <dfn id="queue-a-fetch-request-done-task">queue a fetch-request-done task</dfn>, given a <var>request</var>,
<a href="#queue-a-fetch-task">queue a fetch task</a> on <var>request</var> to <a href="#process-request-end-of-file">process request end-of-file</a>
for <var>request</var>.
<hr>
<p>To <dfn id="read-a-request">read a <var>request</var></dfn>, if <var>request</var>'s
<a href="#concept-request-body" title="concept-request-body">body</a> is non-null, whenever
<var>request</var>'s <a href="#concept-request-body" title="concept-request-body">body</a> is read from (i.e.
is transmitted or read by script), increase <var>request</var>'s
<a href="#concept-request-body" title="concept-request-body">body</a>'s
<a href="#concept-body-transmitted" title="concept-body-transmitted">transmitted bytes</a> with the amount of payload body
bytes transmitted and then <a href="#queue-a-fetch-task">queue a fetch task</a> on <var>request</var> to
<a href="#process-request-body">process request body</a> for <var>request</var>.
<!-- XXX xref "read", "payload body" -->
<h3 id="http"><span class="secno">3.1 </span>HTTP</h3>
<p>While <a href="#concept-fetch" title="concept-fetch">fetching</a> encompasses more than just HTTP, it
borrows a number of concepts from HTTP and applies these to resources obtained via other
means (e.g., <code title="">data</code> URLs).
<p>The <dfn id="http-whitespace-bytes">HTTP whitespace bytes</dfn> are 0x09, 0x0A, 0x0D, and 0x20.
<h4 id="methods"><span class="secno">3.1.1 </span>Methods</h4>
<p>A <dfn id="concept-method" title="concept-method">method</dfn> is a byte sequence that matches the
<a class="external" data-anolis-spec="http" href="https://tools.ietf.org/html/rfc7230#section-3.1.1">method</a> token production.
<p>A <dfn id="simple-method">simple method</dfn> is a <a href="#concept-method" title="concept-method">method</a> that is
`<code title="">GET</code>`, `<code title="">HEAD</code>`, or `<code title="">POST</code>`.
<p>A <dfn id="forbidden-method">forbidden method</dfn> is a <a href="#concept-method" title="concept-method">method</a> that is a
<a href="#byte-case-insensitive">byte-case-insensitive</a> match for `<code title="">CONNECT</code>`,
`<code title="">TRACE</code>`, or `<code title="">TRACK</code>`.
<a class="informative" href="#refsHTTPVERBSEC">[HTTPVERBSEC]</a>
<p>To <dfn id="concept-method-normalize" title="concept-method-normalize">normalize</dfn> a
<a href="#concept-method" title="concept-method">method</a>, if it is a <a href="#byte-case-insensitive">byte-case-insensitive</a>
match for `<code title="">DELETE</code>`, `<code title="">GET</code>`,
`<code title="">HEAD</code>`, `<code title="">OPTIONS</code>`, `<code title="">POST</code>`, or
`<code title="">PUT</code>`, <a href="#byte-uppercase">byte-uppercase</a> it.
<p class="note no-backref"><a href="#concept-method-normalize" title="concept-method-normalize">Normalization</a> is
done for backwards compatibility and consistency across APIs as
<a href="#concept-method" title="concept-method">methods</a> are actually "case-sensitive".
<h4 id="terminology-headers"><span class="secno">3.1.2 </span>Headers</h4>
<p>A <dfn id="concept-header-list" title="concept-header-list">header list</dfn> consists of zero or more
<a href="#concept-header" title="concept-header">headers</a>.
<p class="note no-backref">A <a href="#concept-header-list" title="concept-header-list">header list</a> is essentially a
specialized multimap. An ordered list of key-value pairs with potentially duplicate keys.
<p>To <dfn id="concept-header-list-append" title="concept-header-list-append">append</dfn> a
<a href="#concept-header-name" title="concept-header-name">name</a>/<a href="#concept-header-value" title="concept-header-value">value</a>
(<var>name</var>/<var>value</var>) pair to a
<a href="#concept-header-list" title="concept-header-list">header list</a> (<var>list</var>), append a new
<a href="#concept-header" title="concept-header">header</a> whose <a href="#concept-header-name" title="concept-header-name">name</a>
is <var>name</var>, <a href="#byte-lowercase" title="byte-lowercase">byte-lowercased</a>, and
<a href="#concept-header-value" title="concept-header-value">value</a> is <var>value</var>, to
<var>list</var>.
<p>To <dfn id="concept-header-list-delete" title="concept-header-list-delete">delete</dfn> a
<a href="#concept-header-name" title="concept-header-name">name</a> (<var>name</var>) from a
<a href="#concept-header-list" title="concept-header-list">header list</a> (<var>list</var>), remove all
<a href="#concept-header" title="concept-header">headers</a> whose
<a href="#concept-header-name" title="concept-header-name">name</a> is <var>name</var>,
<a href="#byte-lowercase" title="byte-lowercase">byte-lowercased</a>, from <var>list</var>.
<p>To <dfn id="concept-header-list-set" title="concept-header-list-set">set</dfn> a
<a href="#concept-header-name" title="concept-header-name">name</a>/<a href="#concept-header-value" title="concept-header-value">value</a>
(<var>name</var>/<var>value</var>) pair in a
<a href="#concept-header-list" title="concept-header-list">header list</a> (<var>list</var>), run these
steps:
<ol>
<li><p><a href="#byte-lowercase">Byte-lowercase</a> <var>name</var>.
<li><p>If there are any <a href="#concept-header" title="concept-header">headers</a> in
<var>list</var> whose <a href="#concept-header-name" title="concept-header-name">name</a> is
<var>name</var>, set the <a href="#concept-header-value" title="concept-header-value">value</a> of the first
such <a href="#concept-header" title="concept-header">header</a> to <var>value</var> and remove the
others.
<li><p>Otherwise, append a new <a href="#concept-header" title="concept-header">header</a> whose
<a href="#concept-header-name" title="concept-header-name">name</a> is <var>name</var> and
<a href="#concept-header-value" title="concept-header-value">value</a> is <var>value</var>, to
<var>list</var>.
</ol>
<p>To <dfn id="concept-header-list-combine" title="concept-header-list-combine">combine</dfn> a
<a href="#concept-header-name" title="concept-header-name">name</a>/<a href="#concept-header-value" title="concept-header-value">value</a>
(<var>name</var>/<var>value</var>) pair in a
<a href="#concept-header-list" title="concept-header-list">header list</a> (<var>list</var>), run these
steps:
<ol>
<li><p><a href="#byte-lowercase">Byte-lowercase</a> <var>name</var>.
<li><p>If there are any <a href="#concept-header" title="concept-header">headers</a> in <var>list</var> whose
<a href="#concept-header-name" title="concept-header-name">name</a> is <var>name</var>, set the
<a href="#concept-header-value" title="concept-header-value">value</a> of the first such
<a href="#concept-header" title="concept-header">header</a> to its <a href="#concept-header-value" title="concept-header-value">value</a>,
followed by 0x2C 0x20, followed by <var>value</var>.
<li><p>Otherwise, append a new <a href="#concept-header" title="concept-header">header</a> whose
<a href="#concept-header-name" title="concept-header-name">name</a> is <var>name</var> and
<a href="#concept-header-value" title="concept-header-value">value</a> is <var>value</var>, to
<var>list</var>.
</ol>
<p class="note no-backref"><a href="#concept-header-list-combine" title="concept-header-list-combine">Combine</a> is used by
<a href="https://xhr.spec.whatwg.org/#xmlhttprequest"><code class="external" data-anolis-spec="xhr">XMLHttpRequest</code></a> and the
<a href="#concept-websocket-establish" title="concept-websocket-establish">WebSocket protocol handshake</a>.
<hr>
<p>A <dfn id="concept-header" title="concept-header">header</dfn> consists of a
<dfn id="concept-header-name" title="concept-header-name">name</dfn> and <dfn id="concept-header-value" title="concept-header-value">value</dfn>.
A <a href="#concept-header-name" title="concept-header-name">name</a> is a
<a href="#case-insensitive-byte-sequence">case-insensitive byte sequence</a> that matches the
<a class="external" data-anolis-spec="http" href="https://tools.ietf.org/html/rfc7230#section-3.2">field-name</a> token production. A
<a href="#concept-header-value" title="concept-header-value">value</a> is a byte sequence that matches the
<a class="external" data-anolis-spec="http" href="https://tools.ietf.org/html/rfc7230#section-3.2">field-content</a> token production.
<p class="note no-backref"><a class="external" data-anolis-spec="http" href="https://tools.ietf.org/html/rfc7230#section-3.2">field-value</a> allows 0x0A and
0x0D bytes which can lead to reparsing issues.
<p>To <dfn id="concept-header-value-normalize" title="concept-header-value-normalize">normalize</dfn> a
<a href="#concept-header-value" title="concept-header-value">value</a>, remove any leading and trailing
<a href="#http-whitespace-bytes">HTTP whitespace bytes</a> from it.
<p>A <dfn id="concept-header-value-combined" title="concept-header-value-combined">combined value</dfn>, given a
<a href="#concept-header-name" title="concept-header-name">name</a> (<var>name</var>) and
<a href="#concept-header-list" title="concept-header-list">header list</a> (<var>list</var>), is the
<a href="#concept-header-value" title="concept-header-value">values</a> of all <a href="#concept-header" title="concept-header">headers</a> in
<var>list</var> whose <a href="#concept-header-name" title="concept-header-name">name</a> is <var>name</var>, separated from
each other by `<code title="">,</code>`, in order.
<hr>
<p>A <dfn id="simple-header">simple header</dfn> is a <a href="#concept-header" title="concept-header">header</a> whose
<a href="#concept-header-name" title="concept-header-name">name</a> is one of
<ul class="brief">
<li>`<code title="">Accept</code>`
<li>`<code title="">Accept-Language</code>`
<li>`<code title="">Content-Language</code>`
<li>`<code title="">Content-Type</code>` and whose <a href="#concept-header-value" title="concept-header-value">value</a>,
<a href="#concept-header-parse" title="concept-header-parse">once parsed</a>, has a MIME type (ignoring parameters)
that is `<code title="">application/x-www-form-urlencoded</code>`,
`<code title="">multipart/form-data</code>`, or `<code title="">text/plain</code>`
</ul>
<!-- XXX * needs better xref
* ignoring parameters has been the standard for a long time now
* interesting test: "Content-Type: text/plain;" -->
<p>or whose <a href="#concept-header-name" title="concept-header-name">name</a> is one of
<ul class="brief">
<li>`<code title=""><a href="http://httpwg.org/http-extensions/client-hints.html#dpr">DPR</a></code>`
<li>`<code title=""><a href="http://httpwg.org/http-extensions/client-hints.html#downlink">Downlink</a></code>`
<li>`<code title=""><a href="http://httpwg.org/http-extensions/client-hints.html#save-data">Save-Data</a></code>`
<li>`<code title=""><a href="http://httpwg.org/http-extensions/client-hints.html#viewport-width">Viewport-Width</a></code>`
<li>`<code title=""><a href="http://httpwg.org/http-extensions/client-hints.html#width">Width</a></code>`
</ul>
<p>and whose <a href="#concept-header-value" title="concept-header-value">value</a>,
<a href="#concept-header-parse" title="concept-header-parse">once parsed</a>, is not a failure.
<p>A <dfn id="forbidden-header-name">forbidden header name</dfn> is a <a href="#concept-header" title="concept-header">header</a>
<a href="#concept-header-name" title="concept-header-name">name</a> that is one of
<ul class="brief">
<li>`<code title="">Accept-Charset</code>`
<li>`<code title="">Accept-Encoding</code>`
<li>`<code title="">Access-Control-Request-Headers</code>`
<li>`<code title="">Access-Control-Request-Method</code>`
<li>`<code title="">Connection</code>`
<li>`<code title="">Content-Length</code>`
<li>`<code title="">Cookie</code>`
<li>`<code title="">Cookie2</code>`
<li>`<code title="">Date</code>`
<li>`<code title="">DNT</code>`
<li>`<code title="">Expect</code>`
<li>`<code title="">Host</code>`
<li>`<code title="">Keep-Alive</code>`
<li>`<a href="#http-origin"><code title="http-origin">Origin</code></a>`
<li>`<code title="">Referer</code>`
<li>`<code title="">TE</code>`
<li>`<code title="">Trailer</code>`
<li>`<code title="">Transfer-Encoding</code>`
<li>`<code title="">Upgrade</code>`
<li>`<code title="">Via</code>`
</ul>
<p>or a <a href="#concept-header" title="concept-header">header</a> <a href="#concept-header-name" title="concept-header-name">name</a> that
starts with `<code title="">Proxy-</code>` or `<code title="">Sec-</code>` (including being just
`<code title="">Proxy-</code>` or `<code title="">Sec-</code>`).
<p class="note">These are forbidden so the user agent remains in full control over them.
<a href="#concept-header-name" title="concept-header-name">Names</a> starting with `<code title="">Sec-</code>` are
reserved to allow new <a href="#concept-header" title="concept-header">headers</a> to be minted that are safe
from APIs using <a href="#concept-fetch" title="concept-fetch">fetch</a> that allow control over
<a href="#concept-header" title="concept-header">headers</a> by developers, such as
<a href="https://xhr.spec.whatwg.org/#xmlhttprequest"><code class="external" data-anolis-spec="xhr">XMLHttpRequest</code></a>.
<a class="informative" href="#refsXHR">[XHR]</a>
<p>A <dfn id="forbidden-response-header-name">forbidden response-header name</dfn> is a <a href="#concept-header" title="concept-header">header</a>
<a href="#concept-header-name" title="concept-header-name">name</a> that is one of:
<ul class="brief">
<li>`<code title="">Set-Cookie</code>`
<li>`<code title="">Set-Cookie2</code>`
</ul>
<p>A <dfn id="cors-safelisted-response-header-name">CORS-safelisted response-header name</dfn>, given a
<a href="#concept-response-cors-exposed-header-name-list" title="concept-response-cors-exposed-header-name-list">CORS-exposed header-name list</a>
<var>list</var>, is a <a href="#concept-header" title="concept-header">header</a>
<a href="#concept-header-name" title="concept-header-name">name</a> that is one of:
<ul>
<li>`<code title="">Cache-Control</code>`
<li>`<code title="">Content-Language</code>`
<li>`<code title="">Content-Type</code>`
<li>`<code title="">Expires</code>`
<li>`<code title="">Last-Modified</code>`
<li>`<code title="">Pragma</code>`
<li>Any <a href="#concept-header-value" title="concept-header-value">value</a> in <var>list</var> that is not a
<a href="#forbidden-response-header-name">forbidden response-header name</a>.
</ul>
<hr>
<p>To <dfn id="concept-header-parse" title="concept-header-parse">parse a header value</dfn> given a
<a href="#concept-header-name" title="concept-header-name">name</a> (<var>name</var>) and a
<a href="#concept-header" title="concept-header">header</a> or a <a href="#concept-header-list" title="concept-header-list">header list</a>
(<var>headers</var>), run these steps:
<ol>
<li><p>If <var>name</var> is not in <var>headers</var>, return null.
<li>
<p>If the ABNF for <var>name</var> allows a single
<a href="#concept-header" title="concept-header">header</a> and <var>headers</var> contains more than
one, return failure.
<p class="note no-backref">If different error handling is required, extract the desired
<a href="#concept-header" title="concept-header">header</a> first.
<li><p>If parsing all the <a href="#concept-header" title="concept-header">headers</a>
<a href="#concept-header-name" title="concept-header-name">named</a> <var>name</var> in
<var>headers</var>, per the ABNF for <var>name</var>, failed, return failure.
<li><p>Return one or more <a href="#concept-header-value" title="concept-header-value">values</a> resulting from
parsing all the <a href="#concept-header" title="concept-header">headers</a>
<a href="#concept-header-name" title="concept-header-name">named</a> <var>name</var> in
<var>headers</var>, per the ABNF for <var>name</var>.
</ol>
<p>To <dfn id="concept-header-extract-mime-type" title="concept-header-extract-MIME-type">extract a MIME type</dfn> from a
<a href="#concept-header-list" title="concept-header-list">header list</a> (<var>headers</var>), run these
steps:
<ol>
<li><p>Let <var>MIMEType</var> be the result of
<a href="#concept-header-parse" title="concept-header-parse">parsing</a> `<code title="">Content-Type</code>` in
<var>headers</var>.
<li><p>If <var>MIMEType</var> is null or failure, return the empty byte sequence.
<li><p>Return <var>MIMEType</var>,
<a href="#byte-lowercase" title="byte-lowercase">byte-lowercased</a>.
</ol>
<hr>
<p>A <dfn id="default-user-agent-value">default `<code>User-Agent</code>` value</dfn> is a
user-agent-defined <a href="#concept-header-value" title="concept-header-value">value</a> for the
`<code title="http-user-agent">User-Agent</code>` <a href="#concept-header" title="concept-header">header</a>.
<h4 id="statuses"><span class="secno">3.1.3 </span>Statuses</h4>
<p>A <dfn id="concept-status">status</dfn> is a code.
<p>A <dfn id="null-body-status">null body status</dfn> is a <a href="#concept-status">status</a> that is <code>101</code>,
<code>204</code>, <code>205</code>, or <code>304</code>.
<p>An <dfn id="ok-status">ok status</dfn> is any <a href="#concept-status">status</a> in the range <code>200</code> to
<code>299</code>, inclusive.
<p>A <dfn id="redirect-status">redirect status</dfn> is a <a href="#concept-status">status</a> that is <code>301</code>, <code>302</code>,
<code>303</code>, <code>307</code>, or <code>308</code>.
<h4 id="bodies"><span class="secno">3.1.4 </span>Bodies</h4>
<p>A <dfn id="concept-body" title="concept-body">body</dfn> consists of:
<ul>
<li>
<p>A <dfn id="concept-body-stream" title="concept-body-stream">stream</dfn> (a
<a href="#concept-readablestream"><code title="concept-ReadableStream">ReadableStream</code></a> object).
<p class="XXX"><a href="https://github.com/whatwg/streams/issues/379">This might become a
<code>ReadableByteStream</code> object</a>. While the type might change, the behavior specified
will be equivalent since the hypothetical <code title="">ReadableByteStream</code> object will have
the same members as the <a href="#concept-readablestream"><code title="concept-ReadableStream">ReadableStream</code></a> object has today.
<li><p>A <dfn id="concept-body-transmitted" title="concept-body-transmitted">transmitted bytes</dfn> (an integer), initially 0.
<li><p>A <dfn id="concept-body-total-bytes" title="concept-body-total-bytes">total bytes</dfn> (an integer), initially 0.
</ul>
<p>A <a href="#concept-body" title="concept-body">body</a> <var>body</var> is said to be
<dfn id="concept-body-done" title="concept-body-done">done</dfn> if <var>body</var> is null or <var>body</var>'s
<a href="#concept-body-stream" title="concept-body-stream">stream</a> is
<a href="#concept-readablestream-closed" title="concept-ReadableStream-closed">closed</a> or
<a href="#concept-readablestream-errored" title="concept-ReadableStream-errored">errored</a>.
<p>To <dfn id="concept-body-wait" title="concept-body-wait">wait</dfn> for a <a href="#concept-body" title="concept-body">body</a>
<var>body</var>, wait for <var>body</var> to be <a href="#concept-body-done" title="concept-body-done">done</a>.
<p>To <dfn id="concept-body-clone" title="concept-body-clone">clone</dfn> a <a href="#concept-body" title="concept-body">body</a>
<var>body</var>, run these steps:
<ol>
<li><p>Let «<var>out1</var>, <var>out2</var>» be the result of
<a href="#concept-tee-readablestream" title="concept-tee-ReadableStream">teeing</a> <var>body</var>'s
<a href="#concept-body-stream" title="concept-body-stream">stream</a>. Rethrow any exceptions.
<li><p>Set <var>body</var>'s <a href="#concept-body-stream" title="concept-body-stream">stream</a> to <var>out1</var>.
<li><p>Return a <a href="#concept-body" title="concept-body">body</a> whose
<a href="#concept-body-stream" title="concept-body-stream">stream</a> is <var>out2</var> and other members are copied from
<var>body</var>.
</ol>
<p>To <dfn id="handle-content-codings">handle content codings</dfn> given <var>codings</var> and
<var>bytes</var>, run these substeps:
<ol>
<li><p>If <var>codings</var> are not supported, return <var>bytes</var>.
<li><p>Return the result of decoding bytes with the given
<var>codings</var> as explained in HTTP. <a href="#refsHTTP">[HTTP]</a>
</ol>
<!-- XXX no hook in HTTP -->
<h4 id="requests"><span class="secno">3.1.5 </span>Requests</h4>
<p>The input to <a href="#concept-fetch" title="concept-fetch">fetch</a> is a
<dfn id="concept-request" title="concept-request">request</dfn>.
<p>A <a href="#concept-request" title="concept-request">request</a> has an associated
<dfn id="concept-request-method" title="concept-request-method">method</dfn> (a
<a href="#concept-method" title="concept-method">method</a>). Unless stated otherwise it is
`<code title="">GET</code>`.
<p class="note no-backref">This can be updated during redirects to `<code title="">GET</code>` as
described in <a href="#concept-http-fetch" title="concept-http-fetch">HTTP fetch</a>.
<p>A <a href="#concept-request" title="concept-request">request</a> has an associated
<dfn id="concept-request-url" title="concept-request-url">url</dfn> (a <a href="#fetch-url">fetch URL</a>).
<p>A <a href="#concept-request" title="concept-request">request</a> has an associated
<dfn id="local-urls-only-flag">local-URLs-only flag</dfn>. Unless stated otherwise it is unset.
<p>A <a href="#concept-request" title="concept-request">request</a> has an associated
<dfn id="sandboxed-storage-area-urls-flag">sandboxed-storage-area-URLs flag</dfn>. Unless stated otherwise it is unset.
<p>A <a href="#concept-request" title="concept-request">request</a> has an associated
<dfn id="concept-request-header-list" title="concept-request-header-list">header list</dfn> (a
<a href="#concept-header-list" title="concept-header-list">header list</a>). Unless stated otherwise it is empty.
<p>A <a href="#concept-request" title="concept-request">request</a> has an associated
<dfn id="unsafe-request-flag">unsafe-request flag</dfn>. Unless stated otherwise it is unset.
<p class="note no-backref">The <a href="#unsafe-request-flag">unsafe-request flag</a> is set by APIs such as
<a href="#dom-global-fetch"><code title="dom-global-fetch">fetch()</code></a> and
<a href="https://xhr.spec.whatwg.org/#xmlhttprequest"><code class="external" data-anolis-spec="xhr">XMLHttpRequest</code></a> to ensure a
<a href="#cors-preflight-fetch-0">CORS-preflight fetch</a> is done based on the supplied
<a href="#concept-request-method" title="concept-request-method">method</a> and
<a href="#concept-request-header-list" title="concept-request-header-list">header list</a>. It does not free an API from
outlawing <a href="#forbidden-method" title="forbidden method">forbidden methods</a> and
<a href="#forbidden-header-name" title="forbidden header name">forbidden header names</a>.
<p>A <a href="#concept-request" title="concept-request">request</a> has an associated
<dfn id="concept-request-body" title="concept-request-body">body</dfn> (null or a
<a href="#concept-body" title="concept-body">body</a>). Unless stated otherwise it is null.
<p class="note no-backref">This can be updated during redirects to null as described in
<a href="#concept-http-fetch" title="concept-http-fetch">HTTP fetch</a>.
<hr>
<p>A <a href="#concept-request" title="concept-request">request</a> has an associated
<dfn id="concept-request-client" title="concept-request-client">client</dfn> (null or an
<a class="external" data-anolis-spec="html" href="https://html.spec.whatwg.org/multipage/webappapis.html#environment-settings-object">environment settings object</a>).
<p>A <a href="#concept-request" title="concept-request">request</a> has an associated
<dfn id="concept-request-window" title="concept-request-window">window</dfn> ("<code>no-window</code>",
"<code>client</code>", or an
<a class="external" data-anolis-spec="html" href="https://html.spec.whatwg.org/multipage/webappapis.html#environment-settings-object">environment settings object</a> whose
<a class="external" data-anolis-spec="html" href="https://html.spec.whatwg.org/multipage/webappapis.html#global-object">global object</a> is a
<a href="https://html.spec.whatwg.org/multipage/browsers.html#window"><code class="external" data-anolis-spec="html">Window</code></a> object). Unless stated otherwise it is
"<code>client</code>".
<p class="note no-backref">The "<code>client</code>" value is changed to "<code>no-window</code>" or
<a href="#concept-request" title="concept-request">request</a>'s <a href="#concept-request-client" title="concept-request-client">client</a> during
<a href="#concept-fetch" title="concept-fetch">fetching</a>. It provides a convenient way for standards to not have to
explicitly set <a href="#concept-request" title="concept-request">request</a>'s
<a href="#concept-request-window" title="concept-request-window">window</a>.
<p>A <a href="#concept-request" title="concept-request">request</a> has an associated
<dfn id="concept-request-target-browsing-context" title="concept-request-target-browsing-context">target browsing context</dfn> (null or a
<a class="external" data-anolis-spec="html" href="https://html.spec.whatwg.org/multipage/browsers.html#browsing-context">browsing context</a>). Unless stated otherwise it is null.
<p>A <a href="#concept-request" title="concept-request">request</a> has an associated
<dfn id="keep-alive-flag">keep-alive flag</dfn>. Unless stated otherwise it is unset.
<p class="note no-backref">This is used by <code>navigator.sendBeacon</code> and the HTML
<code>img</code> element to outlive the
<a class="external" data-anolis-spec="html" href="https://html.spec.whatwg.org/multipage/webappapis.html#environment-settings-object">environment settings object</a>.
<p>A <a href="#concept-request" title="concept-request">request</a> has an associated
<dfn id="skip-service-worker-flag">skip-service-worker flag</dfn>. Unless stated otherwise it is unset.
<p>A <a href="#concept-request" title="concept-request">request</a> has an associated
<dfn id="concept-request-initiator" title="concept-request-initiator">initiator</dfn>, which is
the empty string,
"<code title="">download</code>",
"<code title="">imageset</code>",
"<code title="">manifest</code>", or
"<code title="">xslt</code>". Unless stated otherwise it is the empty string.
<p class="note no-backref">A <a href="#concept-request" title="concept-request">request</a>'s
<a href="#concept-request-initiator" title="concept-request-initiator">initiator</a> is not particularly granular for
the time being as other specifications do not require it to. It is primarily a
specification device to assist defining CSP and Mixed Content. It is not exposed to
JavaScript. <a href="#refsCSP">[CSP]</a> <a href="#refsMIX">[MIX]</a>
<p>A <a href="#concept-request" title="concept-request">request</a> has an associated
<dfn id="concept-request-type" title="concept-request-type">type</dfn>, which is
the empty string,
"<code title="">audio</code>",
"<code title="">font</code>",
"<code title="">image</code>",
"<code title="">script</code>",
"<code title="">style</code>",
"<code title="">track</code>", or
"<code title="">video</code>". Unless stated otherwise it is the empty string.
<p>A <a href="#concept-request" title="concept-request">request</a> has an associated
<dfn id="concept-request-destination" title="concept-request-destination">destination</dfn>, which is
the empty string,
"<code title="">document</code>",
"<code title="">embed</code>",
"<code title="">font</code>",
"<code title="">image</code>",
"<code title="">manifest</code>",
"<code title="">media</code>",
"<code title="">object</code>",
"<code title="">report</code>",
"<code title="">script</code>",
"<code title="">serviceworker</code>",
"<code title="">sharedworker</code>",
"<code title="">style</code>",
"<code title="">worker</code>", or
"<code title="">xslt</code>". Unless stated otherwise it is the empty string.
<div class="note">
<p>The following table illustrates the relationship between a
<a href="#concept-request" title="concept-request">request</a>'s
<a href="#concept-request-initiator" title="concept-request-initiator">initiator</a>,
<a href="#concept-request-type" title="concept-request-type">type</a>,
<a href="#concept-request-destination" title="concept-request-destination">destination</a>, CSP directives, and features.
<table>
<tr>
<th><a href="#concept-request-initiator" title="concept-request-initiator">Initiator</a>
<th><a href="#concept-request-type" title="concept-request-type">Type</a>
<th><a href="#concept-request-destination" title="concept-request-destination">Destination</a>
<th>CSP directive
<th>Features
<tr>
<td rowspan="16">""
<td rowspan="6">""
<td>"<code title="">report</code>"
<td rowspan="2">?
<td>CSP, NEL reports.
<tr>
<td>"<code title="">document</code>"
<td>HTML's navigate algorithm.
<tr>
<td>"<code title="">document</code>"
<td><code title="">child-src</code>
<td>HTML's <code><iframe></code> and <code><frame></code>
<tr>
<td>""
<td><code title="">connect-src</code>
<td><code>navigator.sendBeacon()</code>, <code>EventSource</code>,
HTML's <code>ping=""</code>, <a href="#dom-global-fetch"><code title="dom-global-fetch">fetch()</code></a>,
<code>XMLHttpRequest</code>, <code>WebSocket</code>, Cache API?
<tr>
<td>"<code title="">object</code>"
<td><code title="">object-src</code>
<td>HTML's <code><object></code>
<tr>
<td>"<code title="">embed</code>"
<td><code title="">object-src</code>
<td>HTML's <code><embed></code>
<tr>
<td>"<code title="">audio</code>"
<td>"<code title="">media</code>"
<td><code title="">media-src</code>
<td>HTML's <code><audio></code>
<tr>
<td>"<code title="">font</code>"
<td>"<code title="">font</code>"
<td><code title="">font-src</code>
<td>CSS' <code>@font-face</code>
<tr>
<td>"<code title="">image</code>"
<td>"<code title="">image</code>"
<td><code title="">img-src</code>
<td>HTML's <code><img src></code>, <code title="">/favicon.ico</code> resource,
SVG's <code><image></code>, CSS' <code>background-image</code>, CSS'
<code>cursor</code>, CSS' <code>list-style-image</code>, …
<tr>
<td rowspan="4">"<code title="">script</code>"
<td>"<code title="">script</code>"
<td><code title="">script-src</code>
<td>HTML's <code><script></code>, <code>importScripts()</code>
<tr>
<td>"<code title="">serviceworker</code>"
<td>?
<td><code title="">navigator.serviceWorker.register()</code>
<tr>
<td>"<code title="">sharedworker</code>"
<td><code title="">child-src</code>
<td><code>SharedWorker</code>
<tr>
<td>"<code title="">worker</code>"
<td><code title="">child-src</code>
<td><code>Worker</code>
<tr>
<td>"<code title="">style</code>"
<td>"<code title="">style</code>"
<td><code title="">style-src</code>
<td>HTML's <code><link rel=stylesheet></code>, CSS' <code>@import</code>
<tr>
<td>"<code title="">track</code>"
<td>"<code title="">media</code>"
<td><code title="">media-src</code>
<td>HTML's <code><track></code>
<tr>
<td>"<code title="">video</code>"
<td>"<code title="">media</code>"
<td><code title="">media-src</code>
<td>HTML's <code><video></code> element
<tr>
<td>"<code title="">download</code>"
<td>""
<td>""
<td>?
<td>HTML's <code>download=""</code>, "Save Link As…" UI
<tr>
<td>"<code title="">imageset</code>"
<td>"<code title="">image</code>"
<td>"<code title="">image</code>"
<td><code title="">img-src</code>
<td>HTML's <code><img srcset></code> and <code><picture></code>
<tr>
<td>"<code title="">manifest</code>"
<td rowspan="2">""
<td>"<code title="">manifest</code>"
<td><code title="">manifest-src</code>
<td>HTML's <code><link rel=manifest></code>
<tr>
<td>"<code title="">xslt</code>"
<td>"<code title="">xslt</code>"
<td><code title="">script-src</code>
<td><code><?xml-stylesheet></code>
</table>
<p>CSP's <code>form-action</code> needs to be a hook directly in HTML's navigate or form
submission algorithm.
<p>CSP will also need to check
<a href="#concept-request" title="concept-request">request</a>'s
<a href="#concept-request-client" title="concept-request-client">client</a>'s
<a class="external" data-anolis-spec="html" href="https://html.spec.whatwg.org/multipage/browsers.html#browsing-context">browsing context</a>'s
<a class="external" data-anolis-spec="html" href="https://html.spec.whatwg.org/multipage/browsers.html#ancestor-browsing-context" title="ancestor browsing context">ancestor browsing contexts</a>
for various CSP directives.
</div>
<hr>
<p>A <a href="#concept-request" title="concept-request">request</a> has an associated
<dfn id="concept-request-priority" title="concept-request-priority">priority</dfn> (null or a user-agent-defined object).
Unless otherwise stated it is null.
<p>A <a href="#concept-request" title="concept-request">request</a> has an associated
<dfn id="concept-request-origin" title="concept-request-origin">origin</dfn>, which is "<code>client</code>" or an
<a class="external" data-anolis-spec="html" href="https://html.spec.whatwg.org/multipage/browsers.html#origin">origin</a>. Unless stated otherwise it is "<code>client</code>".
<p class="note no-backref">"<code>client</code>" is changed to an
<a class="external" data-anolis-spec="html" href="https://html.spec.whatwg.org/multipage/browsers.html#origin">origin</a> during <a href="#concept-fetch" title="concept-fetch">fetching</a>. It
provides a convenient way for standards to not have to set
<a href="#concept-request" title="concept-request">request</a>'s <a href="#concept-request-origin" title="concept-request-origin">origin</a>.
<a href="#concept-request" title="concept-request">Request</a>'s <a href="#concept-request-origin" title="concept-request-origin">origin</a> can be
changed during redirects too.
<p>A <a href="#concept-request" title="concept-request">request</a> has an associated
<dfn id="omit-origin-header-flag">omit-<code>Origin</code>-header flag</dfn>. Unless stated otherwise it is unset.
<p>A <a href="#concept-request" title="concept-request">request</a> has an associated
<dfn id="same-origin-data-url-flag">same-origin data-URL flag</dfn>. Unless stated otherwise it is unset.
<p>A <a href="#concept-request" title="concept-request">request</a> has an associated
<dfn id="concept-request-referrer" title="concept-request-referrer">referrer</dfn>, which is "<code>no-referrer</code>",
"<code>client</code>", or a <a class="external" data-anolis-spec="url" href="https://url.spec.whatwg.org/#concept-url" title="concept-url">URL</a>.
Unless stated otherwise it is "<code>client</code>".
<p class="note no-backref">"<code>client</code>" is changed to "<code>no-referrer</code>" or a
<a class="external" data-anolis-spec="url" href="https://url.spec.whatwg.org/#concept-url" title="concept-url">URL</a> during
<a href="#concept-fetch" title="concept-fetch">fetching</a>. It provides a convenient way for standards to not have to
set <a href="#concept-request" title="concept-request">request</a>'s
<a href="#concept-request-referrer" title="concept-request-referrer">referrer</a>.
<p>A <a href="#concept-request" title="concept-request">request</a> has an associated
<dfn id="concept-request-referrer-policy" title="concept-request-referrer-policy">referrer policy</dfn>, which is a
<a href="#concept-referrer-policy" title="concept-referrer-policy">referrer policy</a>. Unless stated otherwise it is
the empty string.
<p class="note no-backref">This can be used to override a referrer policy associated with
an <a class="external" data-anolis-spec="html" href="https://html.spec.whatwg.org/multipage/webappapis.html#environment-settings-object">environment settings object</a>.
<a href="#refsREFERRER">[REFERRER]</a>
<p>A <a href="#concept-request" title="concept-request">request</a> has an associated
<dfn id="concept-request-client-hints-list" title="concept-request-client-hints-list">client hints list</dfn>, which is a
<a href="#concept-client-hints-list" title="concept-client-hints-list">client-hints list</a>. Unless stated otherwise, it is
the empty list.
<p class="note no-backref">This will be used to override a client hints list associated with
an <a class="external" data-anolis-spec="html" href="https://html.spec.whatwg.org/multipage/webappapis.html#environment-settings-object">environment settings object</a>.
<a href="#refsCLIENT-HINTS">[CLIENT-HINTS]</a>
<p>A <a href="#concept-request" title="concept-request">request</a> has an associated
<dfn id="synchronous-flag">synchronous flag</dfn>. Unless stated otherwise it is unset.
<p>A <a href="#concept-request" title="concept-request">request</a> has an associated
<dfn id="concept-request-mode" title="concept-request-mode">mode</dfn>, which is "<code title="">same-origin</code>",
"<code title="">cors</code>", "<code title="">no-cors</code>", "<code title="">navigate</code>", or
"<code title="">websocket</code>". Unless stated otherwise, it is "<code title="">no-cors</code>".
<p class="note no-backref">Even though the default <a href="#concept-request" title="concept-request">request</a>
<a href="#concept-request-mode" title="concept-request-mode">mode</a> is "<code title="">no-cors</code>", standards are highly
discouraged from using it for new features. It is rather unsafe. "<code title="">navigate</code>" and
"<code title="">websocket</code>" are special values for the HTML Standard.
<a href="#refsHTML">[HTML]</a>
<p>A <a href="#concept-request" title="concept-request">request</a> has an associated
<dfn id="use-cors-preflight-flag">use-CORS-preflight flag</dfn>. Unless stated otherwise, it is unset.
<p>A <a href="#concept-request" title="concept-request">request</a> has an associated
<dfn id="concept-request-credentials-mode" title="concept-request-credentials-mode">credentials mode</dfn>, which is
"<code title="">omit</code>", "<code title="">same-origin</code>", or
"<code title="">include</code>". Unless stated otherwise, it is "<code title="">omit</code>".
<p class="note no-backref">When <a href="#concept-request" title="concept-request">request</a>'s
<a href="#concept-request-mode" title="concept-request-mode">mode</a> is "<code title="">navigate</code>", its
<a href="#concept-request-credentials-mode" title="concept-request-credentials-mode">credentials mode</a> is assumed to be
"<code title="">include</code>" and <a href="#concept-fetch" title="concept-fetch">fetch</a> does not currently account
for other values. If <cite>HTML</cite> changes here, this standard will need corresponding changes.
<p>A <a href="#concept-request" title="concept-request">request</a> has an associated
<dfn id="concept-request-use-url-credentials-flag" title="concept-request-use-url-credentials-flag">use-URL-credentials flag</dfn>.
Unless stated otherwise, it is unset.
<p>A <a href="#concept-request" title="concept-request">request</a> has an associated
<dfn id="concept-request-cache-mode" title="concept-request-cache-mode">cache mode</dfn>, which is
"<code title="">default</code>", "<code title="">no-store</code>", "<code title="">reload</code>",
"<code title="">no-cache</code>", or "<code title="">force-cache</code>". Unless stated otherwise, it is
"<code title="">default</code>".
<div class="note no-backref">
<dl>
<dt>"<code title="">default</code>"
<dd><a href="#concept-fetch" title="concept-fetch">Fetch</a> will inspect the HTTP cache on the way to the network.
If there is a fresh response it will be used. If there is a stale response a conditional request
will be created, and a normal request otherwise. It then updates the HTTP cache with the response.
<a href="#refsHTTP">[HTTP]</a>
<dt>"<code title="">no-store</code>"
<dd>Fetch behaves as if there is no HTTP cache at all.
<dt>"<code title="">reload</code>"
<dd>Fetch behaves as if there is no HTTP cache on the way to the network. Ergo, it creates a
normal request and updates the HTTP cache with the response.
<dt>"<code title="">no-cache</code>"
<dd>Fetch creates a conditional request if there is a response in the HTTP cache and a normal
request otherwise. It then updates the HTTP cache with the response.
<dt>"<code title="">force-cache</code>"
<dd>Fetch uses any response in the HTTP cache matching the request, not paying attention to
staleness. If there was no response, it creates a normal request updates the HTTP cache with the
response.
</dl>
<p>If <a href="#concept-request-header-list" title="concept-request-header-list">header list</a> contains a
<a href="#concept-header" title="concept-header">header</a> whose <a href="#concept-header-name" title="concept-header-name">name</a> is one
of
`<code title="">If-Modified-Since</code>`,
`<code title="">If-None-Match</code>`,
`<code title="">If-Unmodified-Since</code>`,
`<code title="">If-Match</code>`, and
`<code title="">If-Range</code>`,
<a href="#concept-fetch" title="concept-fetch">fetch</a> will set
<a href="#concept-request-cache-mode" title="concept-request-cache-mode">cache mode</a> to "<code title="">no-store</code>" if it is
"<code title="">default</code>".
</div>
<p>A <a href="#concept-request" title="concept-request">request</a> has an associated
<dfn id="concept-request-redirect-mode" title="concept-request-redirect-mode">redirect mode</dfn>, which is
"<code title="">follow</code>", "<code title="">error</code>", or "<code title="">manual</code>".
Unless stated otherwise, it is "<code title="">follow</code>".
<p>A <a href="#concept-request" title="concept-request">request</a> has associated
<dfn id="concept-request-integrity-metadata" title="concept-request-integrity-metadata">integrity metadata</dfn> (a string). Unless
stated otherwise, it is the empty string.
<p>A <a href="#concept-request" title="concept-request">request</a> has associated
<dfn id="concept-request-nonce-metadata" title="concept-request-nonce-metadata">cryptographic nonce metadata</dfn> (a string). Unless
stated otherwise, it is the empty string.
<p class="note no-backref">A <a href="#concept-request" title="concept-request">request</a>'s
<a href="#concept-request-url-list" title="concept-request-url-list">cryptographic nonce metadata</a> is generally populated
from a <a href="https://html.spec.whatwg.org/multipage/scripting.html#attr-script-nonce"><code class="external" data-anolis-spec="html" title="attr-script-nonce">nonce</code></a> attribute on the element
responsible for triggering a fetch. It is used by various algorithms in
<a href="#refsCSP">[CSP]</a> to determine whether requests should be blocked in a given context.
<hr>