[cssom] Revert CSSNamespaceRule namespaceURI/prefix to be readonly. Fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=22208

Author: zcorpan

cssom/Overview.bs

@@ -2015,40 +2015,16 @@ margin at-rule, with the following properties:
 The <code>CSSNamespaceRule</code> interface represents an <code>@namespace</code> at-rule.
 
 <pre class=idl>interface CSSNamespaceRule : CSSRule {
-  [TreatNullAs=EmptyString] attribute DOMString namespaceURI;
-  [TreatNullAs=EmptyString] attribute DOMString prefix;
+  readonly attribute DOMString namespaceURI;
+  readonly attribute DOMString prefix;
 };</pre>
 
-The <dfn attribute for=CSSNamespaceRule>namespaceURI</dfn> attribute, on getting, must return the namespace of the
+The <dfn attribute for=CSSNamespaceRule>namespaceURI</dfn> attribute must return the namespace of the
 <code>@namespace</code> at-rule.
 
-On setting, the {{CSSNamespaceRule/namespaceURI}} attribute must run the following steps:
-
-<ol>
- <li>Let <var>parent</var> be the <a for=CSSRule>parent CSS style sheet</a>.
- <li>Let <var>list</var> be the <a for=CSSStyleSheet>CSS rules</a> of <var>parent</var>, or
- null if <var>parent</var> is null.
- <li>If <var>list</var> is not null and contains anything other than
- <code>@import</code> at-rules, and <code>@namespace</code> at-rules,
- <a>throw</a> an {{InvalidStateError}} exception.
- <li>Set the namespace of the <code>@namespace</code> at-rule to the given value.
-</ol>
-
 The <dfn attribute for=CSSNamespaceRule>prefix</dfn> attribute must return the prefix of the <code>@namespace</code> at-rule or the
 empty string if there is no prefix.
 
-On setting, the {{CSSNamespaceRule/prefix}} attribute must run the following steps:
-
-<ol>
- <li>Let <var>parent</var> be the <a for=CSSRule>parent CSS style sheet</a>.
- <li>Let <var>list</var> be the <a for=CSSStyleSheet>CSS rules</a> of <var>parent</var>, or
- null if <var>parent</var> is null.
- <li>If <var>list</var> is not null and contains anything other than
- <code>@import</code> at-rules, and <code>@namespace</code> at-rules,
- <a>throw</a> an {{InvalidStateError}} exception.
- <li>Set the prefix of the <code>@namespace</code> at-rule to the given value.
-</ol>
-
 
 CSS Declarations {#css-declarations}
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