[css-counter-styles] Allow 'system' changes, as long as the algorithm doesn't change.

Author: tabatkins

css-counter-styles/Overview.bs

@@ -1989,7 +1989,7 @@ The <code>CSSCounterStyleRule</code> interface</h3>
 	<pre class='idl'>
 	interface CSSCounterStyleRule : CSSRule {
 	  attribute DOMString name;
-	  readonly attribute DOMString system;
+	  attribute DOMString system;
 	  attribute DOMString symbols;
 	  attribute DOMString additiveSymbols;
 	  attribute DOMString negative;
@@ -2042,18 +2042,21 @@ The <code>CSSCounterStyleRule</code> interface</h3>
 
 			<ol>
 				<li>
-					If the attribute is <code>readonly</code>,
-					do nothing and abort these steps.
+					<a>Parse a list of component values</a> from the value.
 
 				<li>
-					<a>Parse a list of component values</a> from the value.
+					If the returned value is invalid according to the given descriptor's grammar,
+					do nothing and abort these steps.
 
 				<li>
-					If the returned value is valid according to the given descriptor's grammar,
-					set that descriptor to the value.
+					If the attribute being set is <a idl>system</a>,
+					and the new value would change the algorithm used,
+					do nothing and abort these steps.
+					<span class='note'>It's okay to change an aspect of the algorithm,
+						like the <a>first symbol value</a> of a ''fixed'' system.</span>
 
 				<li>
-					Otherwise, do nothing.
+					Set the descriptor to the value.
 			</ol>
 	</dl>
 

css-counter-styles/Overview.html

@@ -2010,7 +2010,7 @@ <h3 class="heading settled heading" data-level=9.2 id=the-csscounterstylerule-in
 
 	<pre class=idl>interface <dfn class=idl-code data-dfn-type=interface data-export="" data-global-name="" id=dom-csscounterstylerule>CSSCounterStyleRule<a class=self-link href=#dom-csscounterstylerule></a></dfn> : <a data-link-type=idl href=http://dev.w3.org/csswg/cssom-1/#cssrule title=cssrule>CSSRule</a> {
   attribute DOMString <dfn class=idl-code data-dfn-for=CSSCounterStyleRule data-dfn-type=attribute data-export="" data-global-name="CSSCounterStyleRule<interface>/name<attribute>" id=dom-csscounterstylerule-name>name<a class=self-link href=#dom-csscounterstylerule-name></a></dfn>;
-  readonly attribute DOMString <dfn class=idl-code data-dfn-for=CSSCounterStyleRule data-dfn-type=attribute data-export="" data-global-name="CSSCounterStyleRule<interface>/system<attribute>" id=dom-csscounterstylerule-system>system<a class=self-link href=#dom-csscounterstylerule-system></a></dfn>;
+  attribute DOMString <dfn class=idl-code data-dfn-for=CSSCounterStyleRule data-dfn-type=attribute data-export="" data-global-name="CSSCounterStyleRule<interface>/system<attribute>" id=dom-csscounterstylerule-system>system<a class=self-link href=#dom-csscounterstylerule-system></a></dfn>;
   attribute DOMString <dfn class=idl-code data-dfn-for=CSSCounterStyleRule data-dfn-type=attribute data-export="" data-global-name="CSSCounterStyleRule<interface>/symbols<attribute>" id=dom-csscounterstylerule-symbols>symbols<a class=self-link href=#dom-csscounterstylerule-symbols></a></dfn>;
   attribute DOMString <dfn class=idl-code data-dfn-for=CSSCounterStyleRule data-dfn-type=attribute data-export="" data-global-name="CSSCounterStyleRule<interface>/additivesymbols<attribute>" id=dom-csscounterstylerule-additivesymbols>additiveSymbols<a class=self-link href=#dom-csscounterstylerule-additivesymbols></a></dfn>;
   attribute DOMString <dfn class=idl-code data-dfn-for=CSSCounterStyleRule data-dfn-type=attribute data-export="" data-global-name="CSSCounterStyleRule<interface>/negative<attribute>" id=dom-csscounterstylerule-negative>negative<a class=self-link href=#dom-csscounterstylerule-negative></a></dfn>;
@@ -2062,18 +2062,21 @@ <h3 class="heading settled heading" data-level=9.2 id=the-csscounterstylerule-in
 
 			<ol>
 				<li>
-					If the attribute is <code>readonly</code>,
-					do nothing and abort these steps.
+					<a data-link-type=dfn href=http://dev.w3.org/csswg/css-syntax-3/#parse-a-list-of-component-values0 title="parse a list of component values">Parse a list of component values</a> from the value.
 
 				<li>
-					<a data-link-type=dfn href=http://dev.w3.org/csswg/css-syntax-3/#parse-a-list-of-component-values0 title="parse a list of component values">Parse a list of component values</a> from the value.
+					If the returned value is invalid according to the given descriptor’s grammar,
+					do nothing and abort these steps.
 
 				<li>
-					If the returned value is valid according to the given descriptor’s grammar,
-					set that descriptor to the value.
+					If the attribute being set is <a data-link-type=idl href=#dom-csscounterstylerule-system title=system>system</a>,
+					and the new value would change the algorithm used,
+					do nothing and abort these steps.
+					<span class=note>It’s okay to change an aspect of the algorithm,
+						like the <a data-link-type=dfn href=#first-symbol-value title="first symbol value">first symbol value</a> of a <a class=css data-link-type=maybe href=#valuedef-fixed title=fixed>fixed</a> system.</span>
 
 				<li>
-					Otherwise, do nothing.
+					Set the descriptor to the value.
 			</ol>
 	</dl>