Working with HTML5 at the moment is a lot like the "good old days" of designing pages to work in IE, then redoing the page for Navigator (oooh, showing my age in the industry there!)
But then I copied the HTML into a page being served up through IIS, and accessed via an "http://..." address. Looking at this new page, IE9 now tells me that the canvas element is not supported (modernizr adding the "no-canvas" class to the html element). Weird. Much playing later, found that it is necessary to add the following header metatag to the page:
<meta http-equiv="X-UA-Compatible" content="IE=Edge"/>
Seems that IE treats pages accessed from the file-system differently that those from an http address!