[css-images] Allow either order of <percentage> and <image> in cross-fade().

--HG--
extra : rebase_source : 4a2c84fe7eece6acf11974a98d8d984ea6278c0c

Author: tabatkins

css-images/Overview.html

@@ -951,7 +951,7 @@ <h3 class="heading settled heading" data-level=3.5 id=cross-fade-function><span
 
 <p>	The syntax for <a class=css data-link-type=maybe href=#funcdef-cross-fade title=cross-fade()>cross-fade()</a> is defined as:
 
-<pre class=prod><dfn class=css-code data-dfn-type=function data-export="" id=funcdef-cross-fade title=cross-fade()>cross-fade()<a class=self-link href=#funcdef-cross-fade></a></dfn> = cross-fade( <a class="production css-code" data-link-type=type href=http://dev.w3.org/csswg/css-values-3/#percentage-value title="<percentage>">&lt;percentage&gt;</a>? <a class="production css-code" data-link-type=type href=#typedef-image title="<image>">&lt;image&gt;</a> [, <a class="production css-code" data-link-type=type href=#typedef-image title="<image>">&lt;image&gt;</a> | <a class="production css-code" data-link-type=type href=http://dev.w3.org/csswg/css-color-3/#valuea-def-color title="<color>">&lt;color&gt;</a> ]? )</pre>
+<pre class=prod><dfn class=css-code data-dfn-type=function data-export="" id=funcdef-cross-fade title=cross-fade()>cross-fade()<a class=self-link href=#funcdef-cross-fade></a></dfn> = cross-fade( [ <a class="production css-code" data-link-type=type href=http://dev.w3.org/csswg/css-values-3/#percentage-value title="<percentage>">&lt;percentage&gt;</a>? &amp;&amp; <a class="production css-code" data-link-type=type href=#typedef-image title="<image>">&lt;image&gt;</a> ] [, <a class="production css-code" data-link-type=type href=#typedef-image title="<image>">&lt;image&gt;</a> | <a class="production css-code" data-link-type=type href=http://dev.w3.org/csswg/css-color-3/#valuea-def-color title="<color>">&lt;color&gt;</a> ]? )</pre>
 <p>	The function represents an image generated by
 	combining two images.
 

css-images/Overview.src.html

@@ -941,7 +941,7 @@ <h3 id='cross-fade-function'>
 
 	The syntax for ''cross-fade()'' is defined as:
 
-	<pre class=prod><dfn>cross-fade()</dfn> = cross-fade( <<percentage>>? <<image>> [, <<image>> | <<color>> ]? )</pre>
+	<pre class=prod><dfn>cross-fade()</dfn> = cross-fade( [ <<percentage>>? && <<image>> ] [, <<image>> | <<color>> ]? )</pre>
 
 	The function represents an image generated by
 	combining two images.