[cssom] Don't escape --foo when serializing identifier. Fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=25426

Author: Simon Pieters

cssom/Overview.html

@@ -337,9 +337,6 @@ <h3 id="common-serializing-idioms"><span class="secno">3.1 </span>Common Seriali
  (U+0030 to U+0039) and the first character is a "<code title="">-</code>"
  (U+002D), then the character
  <a href="#escape-a-character-as-code-point" title="escape a character as code point">escaped as code point</a>.</li>
- <li>If the character is the second character and is "<code title="">-</code>"
- (U+002D) and the first character is "<code title="">-</code>" as well, then the
- <a href="#escape-a-character" title="escape a character">escaped</a> character.</li>
  <li>If the character is not handled by one of the above rules and is
  greater than or equal to U+0080, is "<code title="">-</code>" (U+002D) or
  "<code title="">_</code>" (U+005F), or is in one of the ranges [0-9] (U+0030 to

cssom/Overview.src.html

@@ -254,9 +254,6 @@ <h3>Common Serializing Idioms</h3>
  (U+0030 to U+0039) and the first character is a "<code title>-</code>"
  (U+002D), then the character
  <span title="escape a character as code point">escaped as code point</span>.</li>
- <li>If the character is the second character and is "<code title>-</code>"
- (U+002D) and the first character is "<code title>-</code>" as well, then the
- <span title="escape a character">escaped</span> character.</li>
  <li>If the character is not handled by one of the above rules and is
  greater than or equal to U+0080, is "<code title>-</code>" (U+002D) or
  "<code title>_</code>" (U+005F), or is in one of the ranges [0-9] (U+0030 to