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The ones that come to mind that are missing right now:
2. contentEditable nodes
3. the body element when designMode is on
In cases 2 and 3, it would be great if the placeholder text could be HTML and be CSS styleable.
Ojan and I talked about this earlier today. I think this is a good idea, and could replace a lot of JS that web authors have to write to get this behavior.
We should consider reorganizing the code so that maybe HTMLInputElement, HTMLTextAreaElement, and other editable elements can inherit from some common class.
It's a good idea, but we really need to be proposing this to HTML5 before we extend placeholder further IMO.
I've taken the liberty of posting this proposal on whatwg: http://lists.whatwg.org/pipermail/whatwg-whatwg.org/2012-May/035615.html