[cssom] Add a “disable a CSS style sheet” definition

Author: sideshowbarker

cssom-1/Overview.bs

@@ -1197,6 +1197,21 @@ To <dfn export>remove a CSS style sheet</dfn>, run these steps:
  <!-- XXX does anything need to happen wrt alternate style sheets? what if the last style sheet with the preferred style sheet set name is removed? -->
 </ol>
 
+To <dfn export>disable a CSS style sheet</dfn>, run these steps:
+
+<ol>
+ <li>Remove the <a>CSS style sheet</a> from the list of <a>document or shadow root CSS style sheets</a>.
+
+ <li>Set the <a>CSS style sheet</a>'s <a for=CSSStyleSheet>parent CSS style sheet</a>,
+ <a for=CSSStyleSheet>owner node</a> and <a for=CSSStyleSheet>owner CSS rule</a> to null.
+
+ <!-- "associated CSS style sheet" is defined in terms of owner node, so we don't need to set it to null explicitly -->
+
+ <li>Set the <a>disabled flag</a>.
+
+ <!-- XXX does anything need to happen wrt alternate style sheets? what if the last style sheet with the preferred style sheet set name is removed? -->
+</ol>
+
 A <dfn export>persistent CSS style sheet</dfn> is a
 <a>CSS style sheet</a> from the <a>document or shadow root CSS style sheets</a>
 whose <a>title</a> is the empty string and whose