[cssom] CSS.escape() serialize U+0000 as \FFFD instead of throwing

Fixes https://lists.w3.org/Archives/Public/www-style/2015Nov/0293.html.

Author: zcorpan

cssom/Overview.bs

@@ -189,8 +189,8 @@ To <dfn export>serialize an identifier</dfn> means to create a string represente
 by the concatenation of, for each character of the identifier:
 
 <ul>
- <li>If the character is NULL (U+0000), then <a>throw</a> an
- {{InvalidCharacterError}} exception and terminate these steps.
+ <li>If the character is NULL (U+0000), then the REPLACEMENT CHARACTER (U+FFFD)
+ <a>escaped as a code point</a>.
  <li>If the character is in the range [\1-\1f] (U+0001 to U+001F) or is U+007F, then the character
  <a>escaped as code point</a>.
  <li>If the character is the first character and is in the range \[0-9]
@@ -218,8 +218,8 @@ below to each character of the given string, followed by
 '"' (U+0022):
 
 <ul>
- <li>If the character is NULL (U+0000), then <a>throw</a> an
- {{InvalidCharacterError}} exception and terminate these steps.
+ <li>If the character is NULL (U+0000), then the REPLACEMENT CHARACTER (U+FFFD)
+ <a>escaped as a code point</a>.
  <li>If the character is in the range [\1-\1f] (U+0001 to U+001F) or is U+007F, the character
  <a>escaped as code point</a>.
  <li>If the character is '"' (U+0022) or "<code>\</code>"
@@ -3002,7 +3002,7 @@ The <code>CSS</code> interface holds useful CSS-related functions that do not be
 };</pre>
 
 The <dfn method for=CSS>escape(<var>ident</var>)</dfn> method must return the result of invoking <a>serialize an identifier</a> of
-<var>ident</var>. Any exceptions thrown must be re-thrown.
+<var>ident</var>.
 
 <div class=example>
  For example, to escape a string for use as part of a selector, the {{CSS/escape()}} method can be used:
@@ -3203,6 +3203,7 @@ Ms2ger,
 Peter Sloetjes,
 Philip Jägenstedt,
 Philip Taylor,
+Richard Gibson,
 Robert O'Callahan,
 Simon Sapin,
 Sjoerd Visscher,