Usually with our users the biggest issue we find graphics performance wise is that they have manually disabled hardware acceleration. I don't think we've seen a single instance of being forced into software rendering for another reason in the wild.
Just yesterday I sent a an example to someone, on their computer they got nothing instead of a couple of nice balls rendered with a fragment shader.
Reason, their browser just dropped into software rendering and instead of progressive rendering, they got a black canvas with pixels slowly coming up.
No idea about the specific card, an integrated one in a Windows 8 white label PC.
Usually with our users the biggest issue we find graphics performance wise is that they have manually disabled hardware acceleration. I don't think we've seen a single instance of being forced into software rendering for another reason in the wild.