> the following instance renders correctly in Mozilla and Opera

what mime type?

If it is served with text/html
 "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
is full of suntax errors, and  /> should produce character data in the
 head, which is another error so

> Is this not a browser problem?

no, whether anything is rendered at all depends on undocumented error
recovery that may or may not occur in the browser.

If it is served as application/xml
then
opera and mozilla will do what you expect, and IE will correctly (more
or less) parse it using msxml xml parser, the problem is not a parsing
one, it's a rendering one, the rendering is of the markup, with
javascript to allow folding the tree, as is done for any XML without an
<?xml-stylesheet link. Again that isn't a bug, it's conformant to the
specifications that it claims to support, it's just unhelpful.

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