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http://dev.w3.org/csswg/cssom-view/#extensions-to-the-document-interface [[ hit testing ]] is not defined in the spec. There is a note slightly lower in the spec but it would be useful, at least for the WebDriver spec, would rely on this part of CSSOM especially "hit testing"
Pointer Events also seem to require a definition to hit testing for their specification. Put pointer in their meeting to this bug so am updating here too. http://www.w3.org/2013/11/19-pointerevents-minutes.html#item03
CSS UI 3 similarly depends on this but leaves it undefined: https://github.com/w3c/csswg-drafts/commit/8eb712420eeb8144a96ff4092d62796184640cd3
We should add a caveat that the hit testing function should not descend into subdocuments. The naive solution is to use the same hit testing function used for detecting which element should receive a click, but descending into cross-origin frames will leak information about the frame's contents via timing information. At least Firefox[0] and IE11 were affected by this. [0]: https://bugzilla.mozilla.org/show_bug.cgi?id=950427