quirks.bs

<pre class='metadata'>
Group: WHATWG
H1: Quirks Mode
Shortname: quirks
Text Macro: TWITTER quirksstandard
Text Macro: LATESTRD 2022-02
Abstract: Quirks Mode defines quirks in CSS and Selectors that are necessary to support for Web browsers for compatibility with the Web.
</pre>

<pre class="biblio">
{
    "INTRINSIC": {
        "authors": [ "David Baron" ],
        "href": "https://dbaron.org/css/intrinsic/",
        "title": "More Precise Definitions of Intrinsic Widths and Table Layout (Proposal)"
    }
}
</pre>

<pre class=anchors>
urlPrefix: https://drafts.csswg.org/css2/; spec:css2
    urlPrefix: visuren.html
        type: dfn; text: block container element
    urlPrefix: conform.html
        type: dfn; text: illegal
        type: dfn; text: replaced element
    urlPrefix: box.html
        type: dfn; text: border box; url: #border-edge
        type: dfn; text: content box; url: #content-edge
urlPrefix: https://dbaron.org/css/intrinsic/
    type: dfn;
        text: min-content width of an inline formatting context; url: #inline-intrinsic-min
        text: outer min-content width of a table cell; url: #autotable
</pre>

<pre class=link-defaults>
spec:css-sizing-3
    type:property; text:box-sizing
    type:value; for:box-sizing; text:border-box
    type:value; for:width; text:auto
    type:value; for:height; text:auto
    type:property; text:min-height
    type:property; text:max-height
spec:css-tables-3
    type:property; text:border-spacing
spec:css-color-4
    type:property; text:color
spec:cssom-1
    type:interface; text:CSS
spec:selectors-4;
    type:selector; text::active
    type:selector; text::hover
    type:dfn; text:type selector
spec:css-page-floats-3
    type:value; text:none
spec:css-backgrounds-3
    type:property; text:border-top-width
    type:property; text:border-bottom-width
    type:property; text:border-right-width
    type:property; text:border-left-width
    type:property; text:border-style
spec:css-box-4
    type:property; text:padding-top
    type:property; text:padding-bottom
    type:property; text:padding-right
    type:property; text:padding-left
    type:property; text:margin-left
    type:property; text:margin-right
    type:property; text:margin-top
    type:property; text:margin-bottom
spec:css-cascade-5
    type:dfn; text:used value
    type:dfn; text:computed value
spec:css-position-3
    type:property; text:position
spec:css-display-4
    type:property; text:display
    type:dfn; text:containing block
    type:dfn; text:initial containing block
</pre>

<style>
 .logo { position:absolute; top:1em; right:1em; width:106px; height:106px; border:1px outset gray; margin-top:-4px; margin-right:-4px }
 div.head .logo > img { top:2px; right:2px; border:1px inset gray }
 @media (max-width: 767px) {
  .logo { width:calc(4em + 6px); height:calc(4em + 6px) }
 }
</style>


<h2 id=introduction>Introduction</h2>

<h3 id=history>History</h3>

<p>Browsers have several rendering modes to render HTML documents. The reason for this is basically
a historical accident. The CSS specification was incompatible with the behavior of existing browsers
which existing Web content relied on. In order to comply with the specification while not breaking
existing content, browsers introduced a new rendering mode (no-quirks mode). Some browsers still had
the shrink-wrapping behavior for images in table cells in their no-quirks mode, and sites started
relying on that, so browsers that implemented the specification's behavior introduced a third mode
(limited-quirks mode). In hindsight, it would have been better to make the default CSS behavior be
compatible with what the existing content relied on and providing opt-ins to different behavior. The
different modes have since gained a few differences outside of CSS.


<h3 id=goals>Goals</h3>

<ul>

 <li>
  <p>Create a specification for rendering old (or indeed new, if they happen to have a particular
  pragma) HTML documents.

  <p class=note>The HTML specification <a
  href="https://html.spec.whatwg.org/multipage/syntax.html#the-initial-insertion-mode">defines</a>
  when a document is set to <a>quirks mode</a>, <a>limited-quirks mode</a> or <a>no-quirks mode</a>.
  [[HTML]]

 <li>

  <p>Remove quirks from implementations that are not needed for Web compatibility.

  <p class=example id=example-removed-quirks>For example, <a
  href="https://bugzilla.mozilla.org/show_bug.cgi?id=648331">Gecko has removed a quirk about list
  item bullet size</a>, and <a
  href="https://bugs.chromium.org/p/chromium/issues/detail?id=369979">Chromium has removed a quirk
  where display was forced to table or inline-table on <code>table</code> elements</a>.</p>

 <li>

  <p>Get interoperability on quirks that <em>are</em> needed for Web compatibility.

  <p class=example id=example-interop-quirks>For example, <a
  href="https://bugzilla.mozilla.org/show_bug.cgi?id=783213">Gecko aligned their implementation of
  the :active and :hover quirk</a> with this specification and thereby achieved closer
  interoperability with other browsers.

 <li>

  <p>Where possible, limit quirks to a fixed set of legacy features so they don't propagate into
  new features.

  <p class=example id=example-limit-quirks>For example, [[#the-hashless-hex-color-quirk]] is limited to a fixed
  set of CSS properties so that the quirk does not apply in SVG features that also accept colors, or in <a
  href="https://bugs.webkit.org/show_bug.cgi?id=153730">CSS gradients where the grammar could
  otherwise become ambiguous</a>.

</ul>

<p class=note>This specification does not enumerate all quirks that currently exist in browsers. A
number of quirks are specified in HTML, DOM, CSSOM and CSSOM View. [[HTML]] [[DOM]] [[CSSOM]]
[[CSSOM-VIEW]] If a quirk is not specified anywhere, it is probably due to the second bullet point
above.


<h2 id=common-infrastructure>Common infrastructure</h2>

<h3 id=conformance>Conformance</h3>

<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", "SHOULD", "SHOULD NOT", "RECOMMENDED", "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. [[!RFC2119]]


<h3 id=terminology>Terminology</h3>

<p>When this specification refers to a "<code>|foo|</code> element", it means an element with the
local name |foo| and having the namespace <code>http://www.w3.org/1999/xhtml</code>.

<p>When this specification refers to a "<code>|foo|</code> attribute", it means an attribute with
the local name |foo| and having no namespace.

<p>The <dfn>document's body element</dfn> is the first <a spec=dom>child</a> of the <a
spec=dom>document element</a> that is a <{body}> element, if there is one, and the <a
spec=dom>document element</a> is an <{html}> element. Otherwise it is null.

<p class=note>The <a>document's body element</a> is different from HTML's <a spec=html>the body
element</a>, since the latter can be a <{frameset}> element.


<h2 id=css>CSS</h2>

<h3 algorithm id=the-hashless-hex-color-quirk>The hashless hex color quirk</h3>

<p>See [[CSS-COLOR-4#quirky-color]].</p>


<h3 id=the-unitless-length-quirk>The unitless length quirk</h3>

<p>See [[CSS-VALUES-4#deprecated-quirky-length]].</p>


<h3 algorithm id=the-line-height-calculation-quirk>The line height calculation quirk</h3>

<p>In <a spec=dom>quirks mode</a> and <a spec=dom>limited-quirks mode</a>, an inline box that
matches the following conditions, must, for the purpose of line height calculation, act as if the
box had a 'line-height' of zero.

<ul>

 <li><p>The 'border-top-width', 'border-bottom-width', 'padding-top' and 'padding-bottom' properties
 have a <a>used value</a> of zero and the box has a <a>vertical writing mode</a>, or the
 'border-right-width', 'border-left-width', 'padding-right' and 'padding-left' properties have a
 <a>used value</a> of zero and the box has a <a>horizontal writing mode</a>.

 <!-- This matches WebKit. Presto and IE9 don't have this [Presto has now implemented this
(CORE-19183)]. Gecko has this rule for all margins, paddings and borders. -->

 <li><p>It either contains no text or it contains only collapsed whitespace.

 <!-- This matches Gecko and WebKit. IE still uses the quirk in "<pre><img> ". Presto still uses the
 quirk in "<p><img> <img>". -->

 <!-- For the purpose of this bullet point, the 'white-space' processing model must not remove a
 space (U+0020) at the end of a line has 'white-space' set to 'normal', 'nowrap', or 'pre-line' as
 each line is laid out. (This matches Firefox but not WebKit. Presto had a bug about a site that
 relied on the Firefox behavior.)-->

</ul>


<h3 id=the-blocks-ignore-line-height-quirk>The blocks ignore line-height quirk</h3>

<p>In <a spec=dom>quirks mode</a> and <a spec=dom>limited-quirks mode</a>, for a <a>block container
element</a> whose content is composed of <a>inline-level</a> elements, the element's 'line-height'
must be ignored for the purpose of calculating the minimal height of line boxes within the element.

<p class=note>This means that the "strut" is not created.</p>


<h3 algorithm id=the-percentage-height-calculation-quirk>The percentage height calculation
quirk</h3>

<p>In <a spec=dom>quirks mode</a>, for the purpose of calculating the 'height' of an element
|element|, if the <a>computed value</a> of the 'position' property of |element| is
''position/relative'' or ''position/static'', the specified value for the 'height' property of
|element| is a <<percentage>>, and |element| does not have a <a>computed value</a> of the 'display'
property that is ''display/table-row'', ''display/table-row-group'', ''display/table-header-group'',
''display/table-footer-group'', ''display/table-cell'' or ''display/table-caption'', the
<a>containing block</a> of |element| must be calculated using the following algorithm, aborting on
the first step that returns a value:

<ol>

 <li><p>Let |element| be the nearest ancestor <a>containing block</a> of |element|, if there is
 one. Otherwise, return the <a>initial containing block</a>.

 <li><p>If |element| has a <a>computed value</a> of the 'display' property that is
 ''display/table-cell'', then return a UA-defined value.

 <li><p>If |element| has a <a>computed value</a> of the 'height' property that is not
 ''height/auto'', then return |element|.

 <li><p>If |element| has a <a>computed value</a> of the 'position' property that is
 ''position/absolute'', or if |element| is a not a <a>block container</a> or a
 <a>table wrapper box</a>, then return |element|.

 <li><p>Jump to the first step.

</ol>

<p class=note>It is at the time or writing undefined how percentage heights inside tables work in
CSS. This specification does not try to specify what to use as the containing block for calculating
percentage heights in tables. Godspeed!

<p class=big-issue>This quirk needs to take writing modes into account.</p>


<h3 algorithm id=the-html-element-fills-the-viewport-quirk>The <{html}> element fills the viewport
quirk</h3>

<p>In <a spec=dom>quirks mode</a>, if the <a spec=dom>document element</a> |element| matches the
following conditions:

<ul>

 <li><p>|element| is an <{html}> element.

 <li><p>The <a>computed value</a> of the 'width' property of |element| is ''width/auto'' and
 |element| has a <a>vertical writing mode</a>, or the <a>computed value</a> of the 'height' property
 of |element| is ''height/auto'' and |element| has a <a>horizontal writing mode</a>.
 [[!CSS-WRITING-MODES-3]]

</ul>

<p>...then |element| must have its <a>border box</a> size in the <a>block flow direction</a> set
using the following algorithm:

<ol>

 <li><p>Let |margins| be sum of the <a>used values</a> of the 'margin-left' and 'margin-right'
 properties of |element| if |element| has a <a>vertical writing mode</a>, otherwise let |margins| be
 the sum of the <a>used values</a> of the 'margin-top' and 'margin-bottom' properties of |element|.

 <li><p>Let |size| be the size of the <a>initial containing block</a> in the <a>block flow
 direction</a> minus |margins|.

 <li><p>Return the bigger value of |size| and the normal <a>border box</a> size the element would
 have according to the CSS specification.

</ol>


<h3 id=the-body-element-fills-the-html-element-quirk>The <{body}> element fills the <{html}> element
quirk</h3>

<p>In <a spec=dom>quirks mode</a>, if the <a>document's body element</a> |body| is not null and if
it matches the following conditions:

<ul>

 <li><p>The <a>computed value</a> of the 'width' property of |body| is ''width/auto'' and |body| has
 a <a>vertical writing mode</a>, or the <a>computed value</a> of the 'height' property of |body| is
 ''height/auto'' and |body| has a <a>horizontal writing mode</a>. [[!CSS-WRITING-MODES-3]]

 <li><p>The <a>computed value</a> of the 'position' property of |body| is not ''position/absolute''
 or ''position/fixed''.

 <li><p>The <a>computed value</a> of the 'float' property of |body| is ''float/none''.

 <li><p>|body| is not an <a>inline-level</a> element.

 <li><p>|body| is not a <a>multi-column spanning element</a>. [[!CSS3-MULTICOL]]

</ul>

<p>...then |body| must have its <a>border box</a> size in the <a>block flow direction</a> set using
the following algorithm:

<ol>

 <li><p>Let |margins| be the sum of the <a>used values</a> of the 'margin-left' and 'margin-right'
 properties of |body| if |body| has a <a>vertical writing mode</a>, otherwise let |margins| be the
 sum of the <a>used values</a> of the 'margin-top' and 'margin-bottom' properties of |body|.

 <li><p>Let |size| be the size of |body|'s parent element's <a>content box</a> in the <a>block flow
 direction</a> minus |margins|.

 <li><p>Return the bigger value of |size| and the normal <a>border box</a> size the element would
 have according to the CSS specification.

</ol>

<p class=big-issue>What should happen if the <{html}> and the <{body}> have different writing modes?


<h3 id=the-table-cell-width-calculation-quirk>The table cell width calculation quirk</h3>

<p>In <a spec=dom>quirks mode</a>, for the purpose of calculating the <a>min-content width of an
inline formatting context</a> for which a table cell |cell| is the <a>containing block</a>, if
|cell| has a <a>computed value</a> of the 'width' property that is ''width/auto'', <{img}> elements
that are <a>inline-level</a> <a>replaced elements</a> in that <a>inline formatting context</a> must
not have a <a>soft wrap opportunity</a> before or after them. [[!CSS-TEXT-3]] [[!INTRINSIC]]</p>

<!-- http://mxr.mozilla.org/mozilla-central/source/layout/generic/nsImageFrame.cpp#2053 -->


<h3 id=the-table-cell-nowrap-minimum-width-calculation-quirk>The table cell nowrap minimum width
calculation quirk</h3>

<p>In <a spec=dom>quirks mode</a>, an element |cell| that matches the following conditions must act
as if it had an <a>outer min-content width of a table cell</a> in the automatic table layout
algorithm that is the bigger value of |cell|'s <a>computed value</a> of the 'width' property and the
<a>outer min-content width of a table cell</a>. [[!INTRINSIC]]

<ul>

 <li><p>|cell| is a <{td}> element or a <{th}> element.

 <li><p>|cell| has a <code>nowrap</code> attribute.

 <li><p>The <a>computed value</a> of the 'width' property of |cell| is a <<length>> that is not
 zero.

</ul>


<h3 id=the-collapsing-table-quirk>The collapsing table quirk</h3>

<p>In <a spec=dom>quirks mode</a>, an element |table| that matches the following conditions must
have a <a>used value</a> of the 'height' property of ''0'' and a <a>used value</a> of the
'border-style' property of ''border-style/none''.

<ul>

 <li><p>|table| has a <a>computed value</a> of the 'display' property that is ''display/table''.

 <li><p>|table| has no child ''display/table-row-group'', ''display/table-header-group'',
 ''display/table-footer-group'' or ''display/table-caption'' box.

 <li><p>|table| has no child ''display/table-column-group'' box that itself has a child
 ''display/table-column'' box.

</ul>


<h3 id=the-text-decoration-doesnt-propagate-into-tables-quirk>The text decoration doesn't propagate
into tables quirk</h3>

<p>In <a spec=dom>quirks mode</a>, text decoration must not propagate
into <{table}> elements.</p>

<!-- http://mxr.mozilla.org/mozilla-central/source/layout/generic/nsTextFrameThebes.cpp#4582 -->


<h3 algorithm id=the-tables-inherit-color-from-body-quirk>The tables inherit color from body
quirk</h3>

<p>In <a spec=dom>quirks mode</a>, the initial value of the 'color'
property must be ''quirk-inherit'', a special value that has no keyword mapping to it.

<p>The <a>computed value</a> of the 'color' property of an element |element| must be calculated
using the following algorithm:

<ol>

 <li><p>If the specified value of the 'color' property of |element| is not
 ''quirk-inherit'', jump to the last step.

 <li><p>If |element| is not a <{table}> element, jump to the last step.

 <li><p>If the <a>document's body element</a> is null, jump to the last step.

 <li><p>Return the <a>used value</a> of the 'color' property of the <a>document's body element</a>.
 Abort these steps.

 <li><p>If the specified value of the 'color' property of |element| is ''quirk-inherit'', let the
 specified value of the 'color' property of |element| be the initial value of the 'color' property
 according to the CSS specification. Return the <a>computed value</a> of the 'color' property of
 |element| as specified in the CSS specification.

</ol>


<!--
<h3 id=the-font-size-calculation-quirk>The font size calculation quirk</h3>

<p class="note">Status: Needs research.

<pre class=XXX>http://mxr.mozilla.org/mozilla-central/source/layout/style/nsStyleUtil.cpp#64
http://trac.webkit.org/browser/trunk/Source/WebCore/css/CSSStyleSelector.cpp?rev=113922#L4920</pre>

Presto and IE (any mode) don't have this quirk.

https://bugzilla.mozilla.org/show_bug.cgi?id=747464
https://bugs.webkit.org/show_bug.cgi?id=84469
https://code.google.com/p/chromium/issues/detail?id=585390
-->


<h3 id=the-table-cell-height-box-sizing-quirk>The table cell height box sizing quirk</h3>

<p>In <a spec=dom>quirks mode</a>, elements that have a <a>computed value</a> of the 'display'
property of ''display/table-cell'' must act as they have <a>used value</a> of the 'box-sizing'
property of ''box-sizing/border-box'', but only for the purpose of the 'height', 'min-height' and
'max-height' properties.


<h2 id=selectors>Selectors</h2>

<h3 algorithm id=the-active-and-hover-quirk>The :active and :hover quirk</h3>

<p>In <a spec=dom>quirks mode</a>, a <a>compound selector</a> |selector| that matches the following
conditions must not match elements that would not also match the '':any-link'' selector.
[[!SELECTORS4]]

<ul>

 <li><p>|selector| uses the '':active'' or '':hover'' <a>pseudo-classes</a>.

 <li><p>|selector| does not use a <a>type selector</a>.

 <li><p>|selector| does not use an <a>attribute selector</a>.

 <li><p>|selector| does not use an <a>ID selector</a>.

 <li><p>|selector| does not use a <a>class selector</a>.

 <li><p>|selector| does not use a <a>pseudo-class</a> selector other than '':active'' and
 '':hover''.

 <li><p>|selector| does not use a <a>pseudo-element</a> selector.

 <li><p>|selector| is not part of an argument to a <a>functional pseudo-class</a> or
 <a>pseudo-element</a>.

</ul>


<h2 class=no-num id=security-privacy>Security and Privacy Considerations</h2>

<p>There are no known security or privacy impacts in this specification.</p>


<h2 class=no-num id=acknowledgments>Acknowledgments</h2>

<p>Thanks to
Anne van Kesteren,
Boris Zbarsky,
Chris Rebert,
Dan Mulvey,
David Baron,
Kang-Hao Lu,
Ms2ger,
Simon Sapin,
and
Tab Atkins
for their useful comments.

<p>Special thanks to Boris Zbarsky and David Baron for documenting <a
href="https://developer.mozilla.org/en-US/docs/Mozilla/Mozilla_quirks_mode_behavior">Mozilla's
quirks in MDN</a>.

<p>This standard is written by Simon Pieters (<a href="https://www.mozilla.org/">Mozilla</a>, <a
href="mailto:zcorpan@gmail.com">zcorpan@gmail.com</a>).