Clarify CORS attribute reflection

Notably, this renames "canonical case" to "canonical keyword", and
defines exactly how it is used. Closes #3685.

This also rearranges the CORS settings attribute definition a bit to
match other enumerated attributes more closely.

Author: domenic

source

@@ -7025,38 +7025,40 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
   <h4>CORS settings attributes</h4>
 
   <p>A <dfn>CORS settings attribute</dfn> is an <span>enumerated attribute</span>. The following
-  table lists the keywords and states for the attribute &mdash; the keywords in the left column map
-  to the states in the cell in the second column on the same row as the keyword.</p>
+  table lists the keywords and states for the attribute &mdash; the states given in the first cell
+  of the rows with keywords give the states to which those keywords map.</p>
 
   <table>
    <thead>
     <tr>
-     <th> Keyword
      <th> State
+     <th> Keywords
      <th> Brief description
    <tbody>
     <tr>
+     <td rowspan=2><dfn data-x="attr-crossorigin-anonymous">Anonymous</dfn>
      <td><dfn><code data-x="attr-crossorigin-anonymous-keyword">anonymous</code></dfn>
-     <td><dfn data-x="attr-crossorigin-anonymous">Anonymous</dfn>
-     <td><span data-x="concept-request">Requests</span> for the element will have their <span
-     data-x="concept-request-mode">mode</span> set to "<code data-x="">cors</code>" and their <span
-     data-x="concept-request-credentials-mode">credentials mode</span> set to "<code
+     <td rowspan=2><span data-x="concept-request">Requests</span> for the element will have their
+     <span data-x="concept-request-mode">mode</span> set to "<code data-x="">cors</code>" and their
+     <span data-x="concept-request-credentials-mode">credentials mode</span> set to "<code
      data-x="">same-origin</code>".
     <tr>
-     <td><dfn><code data-x="attr-crossorigin-use-credentials-keyword">use-credentials</code></dfn>
+     <td>(the empty string)
+    <tr>
      <td><dfn data-x="attr-crossorigin-use-credentials">Use Credentials</dfn>
+     <td><dfn><code data-x="attr-crossorigin-use-credentials-keyword">use-credentials</code></dfn>
      <td><span data-x="concept-request">Requests</span> for the element will have their <span
      data-x="concept-request-mode">mode</span> set to "<code data-x="">cors</code>" and their <span
      data-x="concept-request-credentials-mode">credentials mode</span> set to "<code
      data-x="">include</code>".
   </table>
 
-  <p>The empty string is also a valid keyword, and maps to the <span
-  data-x="attr-crossorigin-anonymous">Anonymous</span> state. The attribute's <i data-x="invalid value default">invalid value default</i> is the <span data-x="attr-crossorigin-anonymous">Anonymous</span> state. For the
-  purposes of <span data-x="reflect">reflection</span>, the canonical case for the <span
-  data-x="attr-crossorigin-anonymous">Anonymous</span> state is the <code
-  data-x="attr-crossorigin-anonymous-keyword">anonymous</code> keyword. The <i data-x="missing value default">missing value default</i>, used when the attribute is omitted, is the <dfn data-x="attr-crossorigin-none">No
-  CORS</dfn> state.</p>
+  <p>The attribute's <i data-x="invalid value default">invalid value default</i> is the <span
+  data-x="attr-crossorigin-anonymous">Anonymous</span> state, and its <i data-x="missing value
+  default">missing value default</i> is the <dfn data-x="attr-crossorigin-none">No CORS</dfn> state.
+  For the purposes of <span data-x="reflect">reflection</span>, the <span>canonical keyword</span>
+  for the <span data-x="attr-crossorigin-anonymous">Anonymous</span> state is the <code
+  data-x="attr-crossorigin-anonymous-keyword">anonymous</code> keyword.</p>
 
   <p>The majority of fetches governed by <span data-x="CORS settings attribute">CORS settings
   attributes</span> will be done via the <span>create a potential-CORS request</span> algorithm.</p>
@@ -7266,19 +7268,23 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
   <p>If a reflecting IDL attribute is a <code data-x="idl-DOMString">DOMString</code> attribute
   whose content attribute is an <span>enumerated attribute</span>, and the IDL attribute is
   <dfn>limited to only known values</dfn>, then, on getting, the IDL attribute must return the
-  conforming value associated with the state the attribute is in (in its canonical case), if any, or
-  the empty string if the attribute is in a state that has no associated keyword value or if the
-  attribute is not in a defined state (e.g. the attribute is missing and there is no <i
-  data-x="missing value default">missing value default</i>). On setting, the content attribute must
-  be set to the specified new value.</p>
+  keyword value associated with the state the attribute is in, if any, or the empty string if the
+  attribute is in a state that has no associated keyword value or if the attribute is not in a
+  defined state (e.g. the attribute is missing and there is no <i data-x="missing value
+  default">missing value default</i>). If there are multiple keyword values for the state, then
+  return the conforming one. If there are multiple conforming keyword values, then one will be
+  designated the <dfn>canonical keyword</dfn>; choose that one. On setting, the content attribute
+  must be set to the specified new value.</p>
 
   <p>If a reflecting IDL attribute is a nullable <code data-x="idl-DOMString">DOMString</code>
   attribute whose content attribute is an <span>enumerated attribute</span>, then, on getting, if
-  the corresponding content attribute is in its <i>missing value default</i> then the IDL attribute
-  must return null, otherwise, the IDL attribute must return the conforming value associated with
-  the state the attribute is in (in its canonical case). On setting, if the new value is null, the
-  content attribute must be removed, and otherwise, the content attribute must be set to the
-  specified new value.</p>
+  the corresponding content attribute is in its <i>missing value default</i> state then the IDL
+  attribute must return null, otherwise, the IDL attribute must return the keyword value associated
+  with the state the attribute is in. If there are multiple keyword values for the state, then
+  return the conforming one. If there are multiple conforming keyword values, then one will be
+  designated the <span>canonical keyword</span>; choose that one. On setting, if the new value is
+  null, the content attribute must be removed, and otherwise, the content attribute must be set to
+  the specified new value.</p>
 
   <p>If a reflecting IDL attribute is a <code data-x="idl-DOMString">DOMString</code> or <code
   data-x="idl-USVString">USVString</code> attribute but doesn't fall into any of the above