[cssom] Make document.styleSheetSets live and always return the same array. https://www.w3.org/Bugs/Public/show_bug.cgi?id=19904

Author: Simon Pieters

cssom/Overview.html

@@ -1303,8 +1303,9 @@ <h4 id="extensions-to-the-document-interface"><span class="secno">6.2.3 </span>E
 this attribute is initially the empty string.</p>
 
 <p>The <dfn id="dom-document-stylesheetsets" title="dom-Document-styleSheetSets"><code>styleSheetSets</code></dfn> attribute must return a read only array of the
-<a href="#css-style-sheet-set-name" title="CSS style sheet set name">CSS style sheet set names</a> of the
-<a href="#css-style-sheet-set" title="CSS style sheet set">CSS style sheet sets</a>, in order of the <a href="#document-css-style-sheets">document CSS style sheets</a>.</p>
+<a href="#css-style-sheet-set-name" title="CSS style sheet set name">CSS style sheet set names</a> of the <a href="#css-style-sheet-set" title="CSS style sheet set">CSS style sheet sets</a>, in order of the
+<a href="#document-css-style-sheets">document CSS style sheets</a>. The array is live; if the <a href="#document-css-style-sheets">document CSS style sheets</a> change, the array must be updated as appropriate. The
+same array must be returned every time.</p>
 
 <p>The <dfn id="dom-document-enablestylesheetsforset" title="dom-Document-enableStyleSheetsForSet"><code>enableStyleSheetsForSet(<var>name</var>)</code></dfn> method must, when invoked, run these
 steps:</p>

cssom/Overview.src.html

@@ -1225,8 +1225,9 @@ <h4>Extensions to the <code title>Document</code> Interface</h4>
 this attribute is initially the empty string.</p>
 
 <p>The <dfn title=dom-Document-styleSheetSets><code>styleSheetSets</code></dfn> attribute must return a read only array of the
-<span title='CSS style sheet set name'>CSS style sheet set names</span> of the
-<span title='CSS style sheet set'>CSS style sheet sets</span>, in order of the <span>document CSS style sheets</span>.</p>
+<span title='CSS style sheet set name'>CSS style sheet set names</span> of the <span title='CSS style sheet set'>CSS style sheet sets</span>, in order of the
+<span>document CSS style sheets</span>. The array is live; if the <span>document CSS style sheets</span> change, the array must be updated as appropriate. The
+same array must be returned every time.</p>
 
 <p>The <dfn title=dom-Document-enableStyleSheetsForSet><code>enableStyleSheetsForSet(<var>name</var>)</code></dfn> method must, when invoked, run these
 steps:</p>