Drop requirement to register meta[name] extensions

The original spirit behind the requirement to register meta[name] extensions was
good, but in practice it has ended up costing a large of maintenance time
without it being clear how much real benefit to end users it has provided.

Many values listed at https://wiki.whatwg.org/wiki/MetaExtensions appear to be
essentially for private use for only the people/organizations who minted them,
without it being clear how much general utility they might have, and without any
evidence of public applications that consume them.

So this change drops the requirement to formally register new meta[name] names,
while still retaining a reference to https://wiki.whatwg.org/wiki/MetaExtensions
informatively—in order to encourage authors to consult that before creating and
using any new meta[name] names.

source

@@ -531,8 +531,7 @@
    can then look for and process.</p></li>
 
    <li><p>Authors can use the <code data-x="meta">&lt;meta name="" content=""></code> mechanism to
-   include page-wide metadata by registering <span data-x="concept-meta-extensions">extensions to
-   the predefined set of metadata names</span>.</p></li>
+   include page-wide metadata.</p></li>
 
    <li><p>Authors can use the <code data-x="attr-hyperlink-rel">rel=""</code> mechanism to annotate
    links with specific meanings by registering <span data-x="concept-rel-extensions">extensions to
@@ -13201,12 +13200,26 @@ interface <dfn>HTMLMetaElement</dfn> : <span>HTMLElement</span> {
 
   <h5>Other metadata names</h5>
 
-  <p><dfn data-x="concept-meta-extensions">Extensions to the predefined set of metadata names</dfn>
-  may be registered in the <a href="https://wiki.whatwg.org/wiki/MetaExtensions">WHATWG Wiki
-  MetaExtensions page</a>. <ref spec=WHATWGWIKI></p>
+  <p>Anyone can create and use their own <dfn data-x="concept-meta-extensions">extensions to the
+  predefined set of metadata names</dfn>. There is no requirement to register such extensions.</p>
 
-  <p>Anyone is free to edit the WHATWG Wiki MetaExtensions page at any time to add a type. These new
-  names must be specified with the following information:</p>
+  <p>However, if the anticipated metadata name is a <span>URL</span>, registering the value as an
+  <span data-x="concept-rel-extensions">extension to the predefined set of link types</span> instead
+  is encouraged.</p>
+
+  <p>Furthermore, if the anticipated metadata name is something expected to have processing
+  requirements in user agents, proposing it as a new <a href="#standard-metadata-names">standard
+  metadata name</a> instead is encouraged—to find out if there's enough interest among user-agent
+  implementors in actually implementing support for it.</p>
+
+  <p>Also, before creating and using a new metadata name, consulting the <a
+  href="https://wiki.whatwg.org/wiki/MetaExtensions">WHATWG Wiki MetaExtensions page</a> is
+  encouraged—to avoid choosing a metadata name that's actually already in use, and to avoid
+  unnecessarily duplicating the purpose of any metadata names that are already in use. <ref
+  spec=WHATWGWIKI></p>
+
+  <p>Anyone is free to edit the WHATWG Wiki MetaExtensions page at any time to add a metadata name.
+  New metadata names can be specified with the following information:</p>
 
   <dl>
 
@@ -13231,8 +13244,8 @@ interface <dfn>HTMLMetaElement</dfn> : <span>HTMLElement</span> {
    <dt>Synonyms</dt>
 
    <dd><p>A list of other names that have exactly the same processing requirements. Authors should
-   not use the names defined to be synonyms, they are only intended to allow user agents to support
-   legacy content. Anyone may remove synonyms that are not used in practice; only names that need to
+   not use the names defined to be synonyms (they are only intended to allow user agents to support
+   legacy content). Anyone may remove synonyms that are not used in practice; only names that need to
    be processed as synonyms for compatibility with legacy content are to be registered in this
    way.</p></dd>
 
@@ -13267,8 +13280,8 @@ interface <dfn>HTMLMetaElement</dfn> : <span>HTMLElement</span> {
     <p>If a metadata name is found to be redundant with existing values, it should be removed and
     listed as a synonym for the existing value.</p>
 
-    <p>If a metadata name is registered in the "proposed" state for a period of a month or more
-    without being used or specified, then it may be removed from the registry.</p>
+    <p>If a metadata name is added in the "proposed" state for a period of a month or more without
+    being used or specified, then it may be removed from the WHATWG Wiki MetaExtensions page.</p>
 
     <p>If a metadata name is added with the "proposed" status and found to be redundant with
     existing values, it should be removed and listed as a synonym for the existing value. If a
@@ -13282,25 +13295,6 @@ interface <dfn>HTMLMetaElement</dfn> : <span>HTMLElement</span> {
 
   </dl>
 
-  <div w-nodev>
-
-  <p>Conformance checkers must use the information given on the WHATWG Wiki MetaExtensions page to
-  establish if a value is allowed or not: values defined in this specification or marked as
-  "proposed" or "ratified" must be accepted, whereas values marked as "discontinued" or not listed
-  in either this specification or on the aforementioned page must be rejected as invalid.
-  Conformance checkers may cache this information (e.g. for performance reasons or to avoid the use
-  of unreliable network connectivity).</p>
-
-  <p>When an author uses a new metadata name not defined by either this specification or the Wiki
-  page, conformance checkers should offer to add the value to the Wiki, with the details described
-  above, with the "proposed" status.</p>
-
-  </div>
-
-  <p>Metadata names whose values are to be <span data-x="URL">URLs</span> must not be proposed or
-  accepted. Links must be represented using the <code>link</code> element, not the <code>meta</code>
-  element.</p>
-
 
 
   <h5>Pragma directives</h5>
@@ -112117,9 +112111,8 @@ if (s = prompt('What is your name?')) {
    unnecessary. Omit it altogether.</p></dd>
 
    <dt><dfn><code data-x="attr-head-profile">profile</code></dfn> on <code>head</code> elements</dt>
-   <dd><p>When used for declaring which <code>meta</code> terms are
-   used in the document, unnecessary; omit it altogether, and <span
-   data-x="concept-meta-extensions">register the names</span>.</p></dd>
+   <dd><p>When used for declaring which <code>meta</code> terms are used in the document,
+   unnecessary; omit it altogether.</p></dd>
    <dd><p>When used for triggering specific user agent behaviors: use
    a <code>link</code> element instead.</p></dd>