It's not time for HTML5 yet

According to the W3C, HTML5 is currently an unstable, error-prone standard.

The HTML5 standard is not yet ready for "live deployment", said Le Hégaret, one of the leading employees of the W3C (World Wide Web consortium). The consortium does not yet recommend web developers to use HTML5 on the open web, because it is currently not mature enough and therefore suffers from stability and compatibility issues. The warning is not completely unfounded, as the new standard raises several open questions, where a common position is still waiting to be reached.
Implementation of HTML5 is ongoing, however, analysts say it could take 2-3 years for the standard to be approved. It takes approximately that amount of time for web browser developers to fully prepare for the new technology.
Microsoft has previously indicated that it will continue to develop Silverlight independent of the rise of HTML5. With Flash 3D, even Adobe can consolidate the future of the plugin.