http://blogs.msdn.com/b/opal/archive/2010/09/16/ie9-sharepoint-2010-html5.aspx
Disclaimer: This is not a supportability claim for using IE9 on SharePoint 2010. Browser support status will be updated here: http://technet.microsoft.com/en-us/library/cc262485.aspx once the testing is done for specific release version of browsers. Using beta software will never be supported unless you have a special support agreement signed with Microsoft like the “Go-Live” license we did during SP2010 TAP program.
Previously in What’s the Story for HTML5 with SharePoint 2010 we mentioned that HTML5 content can be loaded in to SharePoint pages and libraries. Now Internet Explorer 9 Beta has been released, can it be used to view HTML5 content on SharePoint?
Yes – but we need to do a little bit modification. When you visit a SharePoint 2010 site with IE9, you can open developer tools by pressing F12:
You can find that it is rendered in IE9 7 Compat View, with Document Mode IE8 standards. IE9 is different than Firefox/Safari/Chrome. To ensure the max compatibility, it switches modes when visiting different websites. Unless you specified HTML5 doctype in the page, it will not render <video> and <canvas> tags. So what we need to do in addition is two items:
1. Change doctype in master page (v4.master) to <!DOCTYPE html>
2. Remove <meta http-equiv=”X-UA-Compatible” content=”IE=8”> . This meta tag will force IE9 to act like IE8 and no HTML5 render will be available.
After these are done your HTML5 content should be able to be rendered correctly. The supportability of these method is unknown, so you should only use it with publishing pages since you can control the whole style by yourself.