An update to the “When can I use” page, now you can filter by spec status. This is helpful to only see support for full W3 recommendations, mostly stable specs, features still in flux or completely unofficial features.
I find it interesting to see that with IE8 Microsoft has focused largely on improving items under “candidate recommendation” status (primarily CSS 2.1), whereas most full recommendations (XHTML, SVG, MathML) get ignored. Working draft features seem to only be added when the spec is pretty clear cut, such as the querySelector one.
I’ve decided for now not to add unofficial Microsoft features to the page, since it seems there’s little interest by other browsers to add them or by the W3/WhatWG to standardize them. This is unlike the many features proposed and implemented by Webkit, which have been received positively by other browser vendors. Gecko has even started implementing their Transforms proposal themselves. If Internet Explorer starts adding features that do have browser-wide potential, however, I’ll be glad to mention them too.