[cssom] Give advice for specs that define CSS.foo() functions

Fixes w3c#180.

Author: zcorpan

cssom/Overview.bs

@@ -105,6 +105,8 @@ spec:css-position-3; type:property; text:left
 spec:html5; type:element; text:style
 spec:css-namespaces-3; type:dfn; text:namespace prefix
 spec:dom; type:interface; text:Document
+spec:html; type:dfn; text:ascii case-insensitive
+spec:html; type:dfn; text:case-sensitive
 </pre>
 
 <script src=https://resources.whatwg.org/file-issue.js defer data-file-issue-url="https://github.com/w3c/csswg-drafts/issues/new?title=%5Bcssom%5D%20"></script>
@@ -2648,6 +2650,11 @@ The <dfn method for=CSS>escape(<var>ident</var>)</dfn> method must return the re
  <pre>var element = document.querySelector('a[href="#' + CSS.escape(fragment) + '"]');</pre>
 </div>
 
+Specifications that define static functions on the {{CSS}} interface and want to
+store some state should store the state on the <a spec=html>current global
+object</a>'s <a spec=html>associated <code>Document</code></a>.
+<!-- https://github.com/w3c/csswg-drafts/issues/180 -->
+
 
 Resolved Values {#resolved-values}
 ==================================