[css-shadow-parts-1] Add microsyntaxes for parsing.

Author: fergald

css-shadow-parts-1/Overview.bs

@@ -317,3 +317,62 @@ The token set of this particular DOMTokenList object are also known as the eleme
 
 Issue(w3c/csswg-drafts#2414): Define this as a superglobal in the DOM spec.
 Issue(w3c/csswg-drafts#2414): Define IDL for structured setting and getting of `exportparts`.
+
+Microsyntaxes for parsing {#parsing}
+==============================================
+
+Rules for parsing part mappings {#parsing-mapping}
+----------------------------------------------
+
+A <span>valid part mapping</span> is a pair of DOMTokens
+separated by a U+003A COLON charater,
+with no other characters (e.g. no space characters).
+The DOMTokens must not contain U+003A COLON or U+002C COMMA characters.
+
+The rules for parsing a part mapping are as follows:
+
+1. Let <var>input</var> be the string being parsed.
+
+1. Let <var>position</var> be a pointer into <var>input</var>, initially pointing at the start of the string.
+
+1. <span>Collect a sequence of code points</span> that are space characters or U+003A COLON characters.
+    This skips past any leading delimiters.
+1. <span>Collect a sequence of code points</span> that are not space characters or U+003A COLON characters,
+    and let <var>first token</var> be the result.
+1. If <var>first token</var> is empty then return error.
+1. <span>Collect a sequence of code points</span> that are space characters.
+1. If the end of the <var>input</var> has been reached, return the pair <var>first token</var>, <var>first token</var>
+1. If character at <var>position</var> is not a U+003A COLON character, return error.
+1. Consume the U+003A COLON character.
+1. <span>Collect a sequence of code points</span> that are space characters.
+1. <span>Collect a sequence of code points</span> that are not space characters or U+003A COLON characters.
+    and let <var>second token</var> be the result.
+1. If <var>second token</var> is empty then return error.
+1. Return the pair <var>first token</var>, <var>second token</var>.
+
+Rules for parsing a list of part mappings {#parsing-mapping-list}
+----------------------------------------------
+
+A <span>valid list of part mappings</span> is a number of valid part mappings
+separated by a U+002C COMMA charater,
+with no other characters (e.g. no space characters).
+
+The rules for parsing a list of part mappings are as follow:
+
+1. Let <var>input</var> be the string being parsed.
+
+1. Let <var>position</var> be a pointer into <var>input</var>, initially pointing at the start of the string.
+
+1. Let <var>mappings</var> be an initially empty list of pairs of DOMTokens.
+    This list will be the result of this algorithm.
+
+1. <span>Collect a sequence of code points</span> that are space characters, U+002C COMMA or U+003A COLON characters.
+    This skips past any leading delimiters.
+
+1. While <var>position</var> is not past the end of <var>input</var>:
+
+  1. <span>Collect a sequence of code points</span> that are not U+002C COMMA characters,
+      and let <var>unparsed mapping</var> be the result.
+  1. Let <var>mapping</var> be the result of parsing <var>unparsed mapping</var> using the <span>rules for parsing part mappings</span>.
+  1. If <var>mapping</var> is an error, continue.
+  1. Append <var>mapping</var> to <var>mappings</var>.