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The <output> element doesn't have many additional semantics on top of a non-semantic element like <div>. All it supports is a special "for" attribute that doesn't require any additional behaviour [1]. IE9 is the first version of IE that doesn't barf on unknown new elements in the DOM, so I suppose they got support for this "for free" without any additional development.

[1] http://dev.w3.org/html5/spec/Overview.html#the-output-elemen...

The only reason I marked it as "supported" is that it's "supported" like like any other purely semantic tag is (e.g. <nav>) It's just in with the forms stuff because it's specifically for content coming out of form calculations. I didn't even test the "for" attribute stuff, but wouldn't be surprised if that's not supported.

I'm guessing their support is just that it defaults to display:block and that you can style it.