As it turns out, IEMobile 6/7 (and presumably anything earlier) doesn’t support classes and ids that are attached to the HTML element. I confirmed this on both IEMobile 6 and 7. Fortunately it looks to be fixed in IEMobile 8 (which makes sense, since it works fine in desktop IE6, which it’s based on).

The consequence of this is that adding an id/class to the html tag will result in the style not being applied to the document:

<!doctype html>
<html id="a" class="b">
	<title>Cascade test</title>
	<style type="text/css" media="screen">
		#a span { color: red; }     /* style isn't applied in IEMobile 6/7 */
		.b span { color: yellow; }  /* style isn't applied in IEMobile 6/7 */

<span>Testing 123</span>