This note was at the end of the Controlling focus with tabindex section:

Note: Over time, user agents have provided differing implementations and levels of support for the tabindex attribute. Examples include: the Gecko documentation on Key-navigable custom DHTML widgets; the documentation for Internet Explorer's tabindex behavior in MSDN: tabindex Property, and Opera simple testcases. The HTML 5 spec documents tabindex extended behavior in Sequential focus navigation ([HTML5]).

We removed it because we are able to reference the HTML 5 spec which currently describes tabindex sufficiently. We may need to consider adding it back in at a later point in time though if HTML 5 does not include the tabindex implementation information or if it is not sufficient for implementers. 

Code snippet:

<p class="note">Note: Over time, user agents have provided differing implementations and levels of support for the <code>tabindex</code> attribute. Examples include: the Gecko documentation on <a href="http://wiki.codetalks.org/wiki/index.php/Docs/Keyboard_navigable_JS_widgets">Key-navigable custom <abbr title="Dynamic Hypertext Markup Language">DHTML</abbr> widgets</a>; the documentation for Internet Explorer's <code>tabindex</code> behavior in <a href="http://msdn.microsoft.com/en-us/library/ms534654%28VS.85%29.aspx">MSDN: <code>tabindex</code> Property</a>, and Opera <a href="http://tc.labs.opera.com/html/global-attributes/tabindex/">simple testcases</a>. The <abbr title="Hypertext Markup Language">HTML</abbr> 5 spec documents <code>tabindex</code> extended behavior in <cite><a href="http://www.w3.org/TR/2009/WD-html5-20090825/#sequential-focus-navigation">Sequential focus navigation</a></cite> ([<cite><a href="#ref_HTML5"><abbr title="Hypertext Markup Language 5">HTML5</abbr></a></cite>]).</p>