A lot of people use "HTML5" in a very vague manner.
One of the principle goals of HTML5 is to fully reverse-engineer and document some of Internet Explorer's behaviour so that the other browsers can duplicate it (even in failure scenarios). So a fair bit of HTML5 is already implemented by Microsoft whether they like it or not.
Some other aspects of HTML5 have indeed been adopted by Microsoft and implemented in IE8.
What's going to "kill" Flash and Silverlight, is specific parts of HTML5 such as audio, video and canvas tags which Microsoft have no interest in supporting as it's against their business interests. Both the need for a Microsoft desktop OS and Silverlight are called into question by useful web standards.
What will (continue to) happen is people using whatever bits of the standards they can and falling back to some mix of Javascript, Proprietary IE features and/or Flash. This is standard operating practice for any serious web development. How do you think Google can use SVG in it's maps when that's not supported by IE?
Falling back can be very expensive though when you start to talk about things like Video & Audio, where the cost to create and maintain different formats of large media assets is high.
Interestingly you can deliver vorbis audio natively, via Java, and via Flash 10 (see https://launchpad.net/fogg), so for audio you only need one format for 99% coverage.
And despite that fact no-one talks about it, Vorbis is actually state of the art, competitive with both AAC and AAC-HE at their respective sweet spots but without the problem of AAC-HE degrading badly on decoders that only support AAC (e.g. Apple stuff).
(Vorbis is also patent clear, used widely by big companies, well supported in hardware, and music seems to have moved away from DRM so it basically trumps every objection made to Theora.)
For video, though people like to talk about H.264 as if it was a single codec, it's actually a family or toolkit designed to go from web video on small portable devices up to Hollywood Blue-Ray on 100" projector televisions. (I've seen people seriously claim Theora is unsuitable as a standard web codec because it doesn't handle film grain as well as H.264. Film grain!) Unfortunately you can't encode one H.264 stream that'll work on everything as encode and decode complexity ramp up harshly as you move up the profiles.
I predict that since the iPhone (and similar) is the hot platform right now, and it doesn't have Flash, you'll see a lot of places following Kroc Camen's lead and encoding twice, one for Theora and one for iPhone compatible H.264 which will then be deliver by Flash for non-Theora platforms.
The kicker here is that iPhone compatible H.264 is far from the best possible quality you could squeeze out for desktop use, but is more than good enough for most folks so the two videos will be basically equivalent in quality.
One of the principle goals of HTML5 is to fully reverse-engineer and document some of Internet Explorer's behaviour so that the other browsers can duplicate it (even in failure scenarios). So a fair bit of HTML5 is already implemented by Microsoft whether they like it or not.
Some other aspects of HTML5 have indeed been adopted by Microsoft and implemented in IE8.
http://a.deveria.com/caniuse/#agents=trident&eras=All...
What's going to "kill" Flash and Silverlight, is specific parts of HTML5 such as audio, video and canvas tags which Microsoft have no interest in supporting as it's against their business interests. Both the need for a Microsoft desktop OS and Silverlight are called into question by useful web standards.
What will (continue to) happen is people using whatever bits of the standards they can and falling back to some mix of Javascript, Proprietary IE features and/or Flash. This is standard operating practice for any serious web development. How do you think Google can use SVG in it's maps when that's not supported by IE?