I don‘t see why not. Two of the editors of the HTML5.1 spec are Microsoft representatives: http://www.w3.org/html/wg/drafts/html/master/

If you have to say they will support HTML5 but only on xyz browsers or OS'es then it's like a gas station saying they support Regular but only for Ford and General Motors cars. The reason I can say that is any _other_ site that claims HTML5, works fine on my Chrome browser on Ubuntu.