Make nested comments a parse error

Nesting comments like <!--<!---->--> is an authoring mistake since
the comment will end on the first -->. Now that -- in comments is
allowed, we need new states to make "<!--" in a comment a parse
error.

source

@@ -98254,14 +98254,21 @@ dictionary <dfn>StorageEventInit</dfn> : <span>EventInit</span> {
 
   <h4>Comments</h4>
 
-  <p><dfn data-x="syntax-comments">Comments</dfn> must start with the four character sequence U+003C
-  LESS-THAN SIGN, U+0021 EXCLAMATION MARK, U+002D HYPHEN-MINUS, U+002D HYPHEN-MINUS (<code
-  data-x="">&lt;!--</code>). Following this sequence, the comment may have <span
-  data-x="syntax-text">text</span>, with the additional restriction that the text must not start
-  with a single U+003E GREATER-THAN SIGN character (&gt;), nor start with a U+002D HYPHEN-MINUS
-  character (-) followed by a U+003E GREATER-THAN SIGN (&gt;) character. Finally, the comment must
-  be ended by the three character sequence U+002D HYPHEN-MINUS, U+002D HYPHEN-MINUS, U+003E
-  GREATER-THAN SIGN (<code data-x="">--&gt;</code>).</p>
+  <p><dfn data-x="syntax-comments">Comments</dfn> must have the following format:</p>
+
+  <ol>
+
+   <li>The string "<code data-x="">&lt;!--</code>".</li>
+
+   <li>Optionally, <span data-x="syntax-text">text</span>, with the additional restriction that the
+   text must not start with the string "<code data-x="">&gt;</code>", nor start with the string
+   "<code data-x="">-></code>", nor contain the string "<code data-x="">&lt;!--</code>", nor end
+   with the string "<code data-x="">&lt;!</code>", nor end with the string "<code
+   data-x="">&lt;!-</code>".</li>
+
+   <li>The string "<code data-x="">--&gt;</code>".</li>
+
+  </ol>
 
 
 <!--HTMLPARSER-->
@@ -101198,8 +101205,12 @@ dictionary <dfn>StorageEventInit</dfn> : <span>EventInit</span> {
 
   <dl class="switch">
 
+   <dt>U+003C LESS-THAN SIGN (&lt;)</dt>
+   <dd>Append the <span>current input character</span> to the comment token's data. Switch to the
+   <span>comment less-than sign state</span>.</dd>
+
    <dt>U+002D HYPHEN-MINUS (-)</dt>
-   <dd>Switch to the <span>comment end dash state</span></dd>
+   <dd>Switch to the <span>comment end dash state</span>.</dd>
 
    <dt>U+0000 NULL</dt>
    <dd><span>Parse error</span>. Append a U+FFFD REPLACEMENT CHARACTER character to the comment
@@ -101215,6 +101226,55 @@ dictionary <dfn>StorageEventInit</dfn> : <span>EventInit</span> {
   </dl>
 
 
+  <h5><dfn>Comment less-than sign state</dfn></h5>
+
+  <p>Consume the <span>next input character</span>:</p>
+
+  <dl class="switch">
+
+   <dt>U+0021 EXCLAMATION MARK (!)</dt>
+   <dd>Append the <span>current input character</span> to the comment token's data. Switch to the
+   <span>comment less-than sign bang state</span>.</dd>
+
+   <dt>U+003C LESS-THAN SIGN (&lt;)</dt>
+   <dd>Append the <span>current input character</span> to the comment token's data.</dd>
+
+   <dt>Anything else</dt>
+   <dd><span>Reconsume</span> in the <span>comment state</span>.</dd>
+
+  </dl>
+
+
+  <h5><dfn>Comment less-than sign bang state</dfn></h5>
+
+  <p>Consume the <span>next input character</span>:</p>
+
+  <dl class="switch">
+
+   <dt>U+002D HYPHEN-MINUS (-)</dt>
+   <dd>Switch to the <span>comment less-than sign bang dash state</span>.</dd>
+
+   <dt>Anything else</dt>
+   <dd><span>Reconsume</span> in the <span>comment state</span>.</dd>
+
+  </dl>
+
+
+  <h5><dfn>Comment less-than sign bang dash state</dfn></h5>
+
+  <p>Consume the <span>next input character</span>:</p>
+
+  <dl class="switch">
+
+   <dt>U+002D HYPHEN-MINUS (-)</dt>
+   <dd><span>Parse error</span>. Switch to the <span>comment end state</span>.</dd>
+
+   <dt>Anything else</dt>
+   <dd><span>Reconsume</span> in the <span>comment end dash state</span>.</dd>
+
+  </dl>
+
+
   <h5><dfn>Comment end dash state</dfn></h5>
 
   <p>Consume the <span>next input character</span>:</p>