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<div id="mydiv"><iframe src=facebook.com...

screenshot("#mydiv"), send to my server

would need some security handling, at least.

There was a DOM2Canvas library at some stage, though it wasn't perfect and it couldn't render images (for security reasons).

Multiple browser engines have the ability to do this, but every time I've asked there's been no intent to ever expose it to webpages/webapps, just extensions.