Indeed you are correct. A quick look at the code reveals many statements like:

 return $XHTML ? qq(<input type="submit" tabindex="$tabindex"$name$val
+$other />)
                  : qq(<input type="submit"$name$val$other>);
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The side effect would be that we might not output xhtml, which we currently try to do, but this would be an easy fix for the tab order. I'm guessing we could still output custom headers that indicate the document is XHTML. Thanks for the suggestion.