Restrict the main element to be used once per document

Multiple main elements are still allowed if all but one have the hidden attribute set.

This also slightly alters the definition of the body element as it effectively encompasses all content of a document, except for some metadata.

Fixes #100.

source

@@ -10741,6 +10741,9 @@ console.assert(image.height === 200);</pre>
    <li><span data-x="text content">Text</span></li>
   </ul>
 
+  <p><dfn>Flow content excluding main</dfn> is <span>flow content</span>, but with no
+  <code>main</code> element descendants.</p>
+
 
   <h6>Sectioning content</h6>
 
@@ -15172,7 +15175,7 @@ interface <dfn>HTMLBodyElement</dfn> : <span>HTMLElement</span> {};
    <dd w-nohtml>Uses <code>HTMLBodyElement</code>.</dd>
   </dl>
 
-  <p>The <code>body</code> element <span>represents</span> the main content of the document.</p>
+  <p>The <code>body</code> element <span>represents</span> the contents of the document.</p>
 
   <p>In conforming documents, there is only one <code>body</code> element. The <code
   data-x="dom-document-body">document.body</code> IDL attribute provides scripts with easy access to
@@ -15247,7 +15250,7 @@ interface <dfn>HTMLBodyElement</dfn> : <span>HTMLElement</span> {};
    <dt><span data-x="concept-element-contexts">Contexts in which this element can be used</span>:</dt>
    <dd>Where <span>flow content</span> is expected.</dd>
    <dt><span data-x="concept-element-content-model">Content model</span>:</dt>
-   <dd><span>Flow content</span>.</dd>
+   <dd><span>Flow content excluding main</span>.</dd>
    <dt><span data-x="concept-element-attributes">Content attributes</span>:</dt>
    <dd><span>Global attributes</span></dd>
    <dt><span data-x="concept-element-dom">DOM interface</span>:</dt>
@@ -15390,7 +15393,7 @@ interface <dfn>HTMLBodyElement</dfn> : <span>HTMLElement</span> {};
    <dt><span data-x="concept-element-contexts">Contexts in which this element can be used</span>:</dt>
    <dd>Where <span>flow content</span> is expected.</dd>
    <dt><span data-x="concept-element-content-model">Content model</span>:</dt>
-   <dd><span>Flow content</span>.</dd>
+   <dd><span>Flow content excluding main</span>.</dd>
    <dt><span data-x="concept-element-attributes">Content attributes</span>:</dt>
    <dd><span>Global attributes</span></dd>
    <dt><span data-x="concept-element-dom">DOM interface</span>:</dt>
@@ -15552,7 +15555,7 @@ interface <dfn>HTMLBodyElement</dfn> : <span>HTMLElement</span> {};
    <dt><span data-x="concept-element-contexts">Contexts in which this element can be used</span>:</dt>
    <dd>Where <span>flow content</span> is expected.</dd>
    <dt><span data-x="concept-element-content-model">Content model</span>:</dt>
-   <dd><span>Flow content</span>, but with no <code>main</code> element descendants.</dd>
+   <dd><span>Flow content excluding main</span>.</dd>
    <dt><span data-x="concept-element-attributes">Content attributes</span>:</dt>
    <dd><span>Global attributes</span></dd>
    <dt><span data-x="concept-element-dom">DOM interface</span>:</dt>
@@ -15729,7 +15732,7 @@ interface <dfn>HTMLBodyElement</dfn> : <span>HTMLElement</span> {};
    <dt><span data-x="concept-element-contexts">Contexts in which this element can be used</span>:</dt>
    <dd>Where <span>flow content</span> is expected.</dd>
    <dt><span data-x="concept-element-content-model">Content model</span>:</dt>
-   <dd><span>Flow content</span>, but with no <code>main</code> element descendants.</dd>
+   <dd><span>Flow content excluding main</span>.</dd>
    <dt><span data-x="concept-element-attributes">Content attributes</span>:</dt>
    <dd><span>Global attributes</span></dd>
    <dt><span data-x="concept-element-dom">DOM interface</span>:</dt>
@@ -16118,7 +16121,8 @@ Space is not the only void</pre>
    <dt><span data-x="concept-element-contexts">Contexts in which this element can be used</span>:</dt>
    <dd>Where <span>flow content</span> is expected.</dd>
    <dt><span data-x="concept-element-content-model">Content model</span>:</dt>
-   <dd><span>Flow content</span>, but with no <code>header</code>, <code>footer</code>, or <code>main</code> element descendants.</dd>
+   <dd><span>Flow content excluding main</span>, but with no <code>header</code> or
+   <code>footer</code> element descendants.</dd>
    <dt><span data-x="concept-element-attributes">Content attributes</span>:</dt>
    <dd><span>Global attributes</span></dd>
    <dt><span data-x="concept-element-dom">DOM interface</span>:</dt>
@@ -16204,7 +16208,8 @@ Space is not the only void</pre>
    <dt><span data-x="concept-element-contexts">Contexts in which this element can be used</span>:</dt>
    <dd>Where <span>flow content</span> is expected.</dd>
    <dt><span data-x="concept-element-content-model">Content model</span>:</dt>
-   <dd><span>Flow content</span>, but with no <code>header</code>, <code>footer</code>, or <code>main</code> element descendants.</dd>
+   <dd><span>Flow content excluding main</span>, but with no <code>header</code> or
+   <code>footer</code> element descendants.</dd>
    <dt><span data-x="concept-element-attributes">Content attributes</span>:</dt>
    <dd><span>Global attributes</span></dd>
    <dt><span data-x="concept-element-dom">DOM interface</span>:</dt>
@@ -16348,10 +16353,9 @@ Space is not the only void</pre>
    <dt><span data-x="concept-element-contexts">Contexts in which this element can be used</span>:</dt>
    <dd>Where <span>flow content</span> is expected.</dd>
    <dt><span data-x="concept-element-content-model">Content model</span>:</dt>
-   <dd><span>Flow content</span>, but with no <span>heading
-   content</span> descendants, no <span>sectioning content</span>
-   descendants, and no <code>header</code>, <code>footer</code>, or
-   <code>address</code> element descendants.</dd>
+   <dd><span>Flow content excluding main</span>, but with no <span>heading content</span>
+   descendants, no <span>sectioning content</span> descendants, and no <code>header</code>,
+   <code>footer</code>, or <code>address</code> element descendants.</dd>
    <dt><span data-x="concept-element-attributes">Content attributes</span>:</dt>
    <dd><span>Global attributes</span></dd>
    <dt><span data-x="concept-element-dom">DOM interface</span>:</dt>
@@ -17959,7 +17963,7 @@ a friend lost to the
    <dt><span data-x="concept-element-contexts">Contexts in which this element can be used</span>:</dt>
    <dd>Where <span>flow content</span> is expected.</dd>
    <dt><span data-x="concept-element-content-model">Content model</span>:</dt>
-   <dd><span>Flow content</span>.</dd>
+   <dd><span>Flow content excluding main</span>.</dd>
    <dt><span data-x="concept-element-attributes">Content attributes</span>:</dt>
    <dd><span>Global attributes</span></dd>
    <dd><code data-x="attr-blockquote-cite">cite</code></dd>
@@ -18877,7 +18881,9 @@ first matching case):&lt;/p&gt;
    <dd>Before <code>dd</code> or <code>dt</code> elements inside <code>dl</code> elements.</dd>
    <dd>Before <code>dd</code> or <code>dt</code> elements inside <code>div</code> elements that are children of a <code>dl</code> element.</dd>
    <dt><span data-x="concept-element-content-model">Content model</span>:</dt>
-   <dd><span>Flow content</span>, but with no <code>header</code>, <code>footer</code>, <span>sectioning content</span>, or <span>heading content</span> descendants.</dd>
+   <dd><span>Flow content excluding main</span>, but with no <code>header</code>,
+   <code>footer</code>, <span>sectioning content</span>, or <span>heading content</span>
+   descendants.</dd>
    <dt><span data-x="concept-element-attributes">Content attributes</span>:</dt>
    <dd><span>Global attributes</span></dd>
    <dt><span data-x="concept-element-dom">DOM interface</span>:</dt>
@@ -18962,9 +18968,11 @@ first matching case):&lt;/p&gt;
    <dt><span data-x="concept-element-contexts">Contexts in which this element can be used</span>:</dt>
    <dd>Where <span>flow content</span> is expected.</dd>
    <dt><span data-x="concept-element-content-model">Content model</span>:</dt>
-   <dd>Either: One <code>figcaption</code> element followed by <span>flow content</span>.</dd>
-   <dd>Or: <span>Flow content</span> followed by one <code>figcaption</code> element.</dd>
-   <dd>Or: <span>Flow content</span>.</dd>
+   <dd>Either: One <code>figcaption</code> element followed by <span>flow content excluding
+   main</span>.</dd>
+   <dd>Or: <span>Flow content excluding main</span> followed by one <code>figcaption</code>
+   element.</dd>
+   <dd>Or: <span>Flow content excluding main</span>.</dd>
    <dt><span data-x="concept-element-attributes">Content attributes</span>:</dt>
    <dd><span>Global attributes</span></dd>
    <dt><span data-x="concept-element-dom">DOM interface</span>:</dt>
@@ -19190,22 +19198,19 @@ included with Exhibit B.
    <dt><span data-x="concept-element-contexts">Contexts in which this element can be used</span>:</dt>
    <dd>Where <span>flow content</span> is expected.</dd>
    <dt><span data-x="concept-element-content-model">Content model</span>:</dt>
-   <dd><span>Flow content</span>.</dd>
+   <dd><span>Flow content excluding main</span>.</dd>
    <dt><span data-x="concept-element-attributes">Content attributes</span>:</dt>
    <dd><span>Global attributes</span></dd>
    <dt><span data-x="concept-element-dom">DOM interface</span>:</dt>
    <dd>Uses <code>HTMLElement</code>.</dd>
   </dl>
 
-  <p>The <code>main</code> element can be used as a container for the dominant contents of the
-  document. It <span>represents</span> its children.</p>
-
-  <p class="note">The <code>main</code> element is distinct from the <code>section</code> and
-  <code>article</code> elements in that the <code>main</code> element does not contribute to the
-  document <span>outline</span>.</p>
+  <p>The <code>main</code> element <span>represents</span> the dominant contents of the
+  document.</p>
 
-  <p class="note">While there is no restriction as to the number of <code>main</code> elements in a
-  document, web developers are encouraged to stick to a single element.</p>
+  <p>A document must not have more than one <code>main</code> element, unless all but one of the
+  <code>main</code> elements in the document have the <code data-x="attr-hidden">hidden</code>
+  attribute specified.</p>
 
   <div class="example">
 
@@ -19247,6 +19252,35 @@ included with Exhibit B.
 &lt;/footer>
 &lt;/html></pre>
 
+   <p>In the following example, multiple <code>main</code> elements are used and script is used to
+   make navigation work without a server roundtrip and to set the <code>hidden</code> attribute on
+   those that are not current:
+
+   <pre>&lt;!doctype html>
+&lt;html lang=en-CA>
+&lt;meta charset=utf-8>
+&lt;title> &hellip; &lt;/title>
+&lt;link rel=stylesheet href=spa.css>
+&lt;script src=spa.js async>&lt;/script>
+&lt;nav>
+ &lt;a href=/>Home&lt;/a>
+ &lt;a href=/about>About&lt;/a>
+ &lt;a href=/contact>Contact&lt;/a>
+&lt;/nav>
+&lt;main>
+ &lt;h1>Home&lt;/h1>
+ &hellip;
+&lt;/main>
+&lt;main hidden>
+ &lt;h1>About&lt;/h1>
+ &hellip;
+&lt;/main>
+&lt;main hidden>
+ &lt;h1>Contact&lt;/h1>
+ &hellip;
+&lt;/main>
+&lt;footer>Made with ❤️ by &lt;a href=https://example.com/>Example 👻&lt;/a>.&lt;/footer></pre>
+
   </div>
 
 
@@ -40519,7 +40553,7 @@ interface <dfn>HTMLTableRowElement</dfn> : <span>HTMLElement</span> {
    <dt><span data-x="concept-element-contexts">Contexts in which this element can be used</span>:</dt>
    <dd>As a child of a <code>tr</code> element.</dd>
    <dt><span data-x="concept-element-content-model">Content model</span>:</dt>
-   <dd><span>Flow content</span>.</dd>
+   <dd><span>Flow content excluding main</span>.</dd>
    <dt><span data-x="concept-element-attributes">Content attributes</span>:</dt>
    <dd><span>Global attributes</span></dd>
    <dd><code data-x="attr-tdth-colspan">colspan</code></dd>
@@ -40613,8 +40647,9 @@ interface <dfn>HTMLTableCellElement</dfn> : <span>HTMLElement</span> {
    <dt><span data-x="concept-element-contexts">Contexts in which this element can be used</span>:</dt>
    <dd>As a child of a <code>tr</code> element.</dd>
    <dt><span data-x="concept-element-content-model">Content model</span>:</dt>
-   <dd><span>Flow content</span>, but with no <code>header</code>, <code>footer</code>,
-   <span>sectioning content</span>, or <span>heading content</span> descendants.</dd>
+   <dd><span>Flow content excluding main</span>, but with no <code>header</code>,
+   <code>footer</code>, <span>sectioning content</span>, or <span>heading content</span>
+   descendants.</dd>
    <!-- EDITING NOTE: If you remove 'heading content' from the list above, make sure to fix the definition of 'sectioning root' to include <th> elements -->
    <dt><span data-x="concept-element-attributes">Content attributes</span>:</dt>
    <dd><span>Global attributes</span></dd>
@@ -42744,7 +42779,8 @@ interface <dfn>HTMLTableCellElement</dfn> : <span>HTMLElement</span> {
    <dt><span data-x="concept-element-contexts">Contexts in which this element can be used</span>:</dt>
    <dd>Where <span>flow content</span> is expected.</dd>
    <dt><span data-x="concept-element-content-model">Content model</span>:</dt>
-   <dd><span>Flow content</span>, but with no <code>form</code> element descendants.</dd>
+   <dd><span>Flow content excluding main</span>, but with no <code>form</code> element
+   descendants.</dd>
    <dt><span data-x="concept-element-attributes">Content attributes</span>:</dt>
    <dd><span>Global attributes</span></dd>
    <dd><code data-x="attr-form-accept-charset">accept-charset</code></dd>
@@ -51983,7 +52019,8 @@ out of 233&#x2009;257&#x2009;824 bytes available&lt;/meter>&lt;/p></pre>
    <dt><span data-x="concept-element-contexts">Contexts in which this element can be used</span>:</dt>
    <dd>Where <span>flow content</span> is expected.</dd>
    <dt><span data-x="concept-element-content-model">Content model</span>:</dt>
-   <dd>Optionally a <code>legend</code> element, followed by <span>flow content</span>.</dd>
+   <dd>Optionally a <code>legend</code> element, followed by <span>flow content excluding
+   main</span>.</dd>
    <dt><span data-x="concept-element-attributes">Content attributes</span>:</dt>
    <dd><span>Global attributes</span></dd>
    <dd><code data-x="attr-fieldset-disabled">disabled</code></dd>
@@ -56381,7 +56418,7 @@ fur
    <dt><span data-x="concept-element-contexts">Contexts in which this element can be used</span>:</dt>
    <dd>Where <span>flow content</span> is expected.</dd>
    <dt><span data-x="concept-element-content-model">Content model</span>:</dt>
-   <dd>One <code>summary</code> element followed by <span>flow content</span>.</dd>
+   <dd>One <code>summary</code> element followed by <span>flow content excluding main</span>.</dd>
    <dt><span data-x="concept-element-attributes">Content attributes</span>:</dt>
    <dd><span>Global attributes</span></dd>
    <dd><code data-x="attr-details-open">open</code></dd>