Mozilla's technical evangelist Paul Rouget tears into Microsoft, claiming that while Internet Explorer 9 is "definitely better than IE8 and a step in the right direction," it's still not a modern browser.
Rouget backs up his claims by pointing out that Microsoft uses tests created during the development of IE9 as a benchmark don't offer a balanced view of the results.
Does IE9 support 99% of the HTML5 specification as insinuated by Microsoft? No, they're actually pretty far from it. The tests Microsoft are referring to are the ones they created during the development of IE9. It's not that surprising that they pass the very tests they used to design and develop the browser - we score pretty well against our own unit-tests as well. The primary use case for these tests, however, is to spot regressions and validate code changes. In other words: the tests ensure that future changes don't break the things you just built. They don't actually test all elements of a specific standard.
Rouget then goes on to pitch IE9 against Firefox on two different HTML5 tests, and the results are interesting:
The reality is that IE9 is 2 years late. Microsoft is glad to come out with the <video> tag, the <canvas> tag, SVG, and some CSS3. Like other vendors did years ago. Firefox 3.5 had the <video> tag, the <canvas> tag, Geolocation, SVG in 2009. Canvas and SVG existed 5 years ago.
Even the guys that wrote these tests came to the same conclusion: Niels Leenheer: ‘Microsoft talks big about HTML5 but shows very little', caniuse.com about IE9RC1: ‘[IE9] being about on par with Firefox 3.6'.
Still, a better version of IE is not a bad thing.