If HTML becomes primary platform for Windows we are back in stone age and there is no point in reimplementing WPF/Silverlight markup by HTML markup as such markup extension will not work on other platforms which is main goal of adopting HTML.

I think that WPF/SIlverlight should be first class platforms for tile mode in Windows 8, but this mode should also support HTML. I believe they will use this approach. Also, they said that Silverlight will still run in the browser which means that we would be able to  embed Silverlight into HTML inside tile apps and probably to build those apps entirely in SIlverlight but the real issue here is that HTML will be primary platform. They said that in order to get the most out of tiles inside Windows 8 you will need HTML/Javascript which is total madness.