Rewrite section for SVG/HTML:

- convert it to examples and referring to HTML/SVG spec instead.
- `requiredExtension` is preserved, as that's in browsers/tests but not in the SVG spec yet.
- extend the annotation-xml example to show one can embed SVG/HTML too.

Closes #195

examples/example-semantics.html

@@ -12,5 +12,32 @@
          <cn>2</cn>
       </apply>
     </annotation-xml>
+    <annotation-xml>
+      <svg width="25" height="75" xmlns="http://www.w3.org/2000/svg">
+        <path stroke-width="5.8743"
+              d="m5.9157 27.415h6.601v-22.783l-7.1813 1.4402v-3.6805l7.1408
+                 -1.4402h4.0406v26.464h6.601v3.4005h-17.203z"/>
+        <path stroke="#000000" stroke-width="2.3409"
+              d="m0.83496 39.228h23.327"/>
+        <path stroke-width="5.8743"
+              d="m8.696 70.638h14.102v3.4005h-18.963v-3.4005q2.3004-2.3804
+                 6.2608-6.3813 3.9806-4.0206 5.0007-5.1808 1.9403-2.1803
+                 2.7004-3.6805 0.78011-1.5202 0.78011-2.9804 0-2.3804
+                 -1.6802-3.8806-1.6603-1.5002-4.3406-1.5002-1.9003 0-4.0206
+                 0.6601-2.1003 0.6601-4.5007 2.0003v-4.0806q2.4404-0.98013
+                 4.5607-1.4802 2.1203-0.50007 3.8806-0.50007 4.6407 0 7.401
+                 2.3203 2.7604 2.3203 2.7604 6.2009 0 1.8403-0.7001 3.5006
+                 -0.68013 1.6402-2.5004 3.8806-0.50007 0.58009-3.1805 3.3605
+                 -2.6804 2.7604-7.5614 7.7412z"/>
+      </svg>
+    </annotation-xml>
+    <annotation-xml encoding="application/xhtml+xml">
+      <div style="display: inline-flex;
+                  flex-direction: column; align-items: center;">
+        <div>1</div>
+        <div>―</div>
+        <div>2</div>
+      </div>
+    </annotation-xml>
   </semantics>
 </math>

spec.html

@@ -576,22 +576,8 @@ <h3>Integration in the Web Platform</h3>
         <section id="html-and-svg">
           <h4>HTML and SVG</h4>
           <p>
-            When
-            <a data-cite="HTML/../#parsing">parsing HTML documents</a>
-            user agents must treat any tag name corresponding to a
-            <a>MathML Core Element</a> as belonging to the
-            <a>MathML namespace</a>.
-          </p>
-          <p>
-            Users agents must allow mixing HTML, SVG and MathML elements
-            as allowed by sections
-            <a data-cite="HTML/../#html-integration-point">HTML integration point</a>,
-            <a data-cite="HTML/../#mathml-text-integration-point">MathML integration point</a>,
-            <a data-cite="HTML/../#tree-construction-dispatcher">tree construction
-              dispatcher</a>,
-            <a data-cite="HTML/../#mathml">MathML</a> and
-            <a data-cite="HTML/../#svg-0">SVG</a>
-            from [[HTML]].
+            MathML can be mixed with HTML and SVG as described in the relevant
+            specifications [[HTML]] [[SVG]].
           </p>
           <p>
             When evaluating the SVG [^svg/requiredExtensions^]
@@ -609,46 +595,14 @@ <h4>HTML and SVG</h4>
               HTML <code>input</code> element is used within the
               [^mtext^]
               to include an interactive input field inside a mathematical
-              formula.
+              formula. See also <a href="#semantics-and-presentation"></a>
+              for an example of SVG and HTML inside an [^annotation-xml^]
+              element.
             </p>
             <pre data-include="examples/example-html-svg.html"
                  data-include-format="text"></pre>
             <img src="examples/example-html-svg.png" alt="html-svg example">
           </div>
-          <div class="note">
-            <ul>
-              <li>
-                The <code>&lt;math&gt;</code> element can be used at
-                position permitted for
-                <a>flow content</a>
-                (e.g. a
-                <a data-cite="SVG/embedded.html#ForeignObjectElement"><code>&lt;foreignObject&gt;</code></a> element)
-                or <a>phrasing content</a>.
-              </li>
-              <li>
-                Any
-                <a>phrasing content</a>
-                can be used inside
-                [^mi^],
-                [^mo^],
-                [^mn^],
-                [^ms^] and
-                [^mtext^]
-                elements.
-              </li>
-              <li>
-                The <code>&lt;svg&gt;</code> element can be used inside
-                [^annotation-xml^] elements.
-              </li>
-              <li>
-                Any <a>flow content</a>
-                can be used inside
-                [^annotation-xml^] elements with
-                encoding
-                <code>application/xhtml+xml</code> or <code>text/html</code>.
-              </li>
-            </ul>
-          </div>
         </section>
         <section id="css-styling">
           <h4>CSS styling</h4>
@@ -4677,8 +4631,9 @@ <h3>Semantics and Presentation</h3>
           <p>
             The following example shows how the fraction "one half" can be
             annotated with a textual annotation (LaTeX) or an XML annotation
-            (content MathML). These annotations are not intended to be rendered
-            by the user agent.
+            (content MathML), which are not intended to be rendered
+            by the user agent. This fraction is also annotated with equivalent
+            SVG and HTML markup.
           </p>
           <pre data-include="examples/example-semantics.html"
                data-include-format="text"></pre>