[whatwg] Drop tabindex=""

[Gervase Markham]

Colin Lieberman wrote:
> Drop tabindex altogether. It's just not useful.

Before doing that, it might make sense to consult the accessibility 
teams of the UA vendors. In Mozilla's case, that's Aaron Leventhal. I 
believe that there have been recent changes to this property to better 
allow keyboard accessibility of DHTML widgets:
http://developer.mozilla.org/en/docs/Key-navigable_custom_DHTML_widgets

Gerv