[[

A personal comment on http://www.w3.org/TR/2011/WD-
html5-20110525/rendering.html#rendering

Chapter 10 seems to say that this CSS style sheet:

    @namespace url(http://www.w3.org/1999/xhtml);
    sup { vertical-align: super; }

should in the future match SUP elements in HTML as well as in XHTML. 
Currently, HTML documents don't have a namespace, and so it is easy to 
distinguish HTML from XHTML in CSS by omitting the namespace:

    h1 {...}    /* HTML and XHTML */
    x|h1 {...}  /* XHTML only (after suitable defn of "x") */
    |h1 {...}   /* HTML only */

CSS has been changed to remove differences between HTML and XHTML, so 
there is now less need to distinguish them, but people may still want 
to. And there probably are existing style sheets with such rules. 
(Several current browsers are buggy and fail to apply "|h1" even now, 
but there are other UAs that are correct.)
]]

EDITOR'S RESPONSE: This is an Editor's Response to your comment. If you are satisfied with this response, please change the state of this bug to CLOSED. If you have additional information and would like the editor to reconsider, please reopen this bug. If you would like to escalate the issue to the full HTML Working Group, please add the TrackerRequest keyword to this bug, and suggest title and text for the tracker issue; or you may create a tracker issue yourself, if you are able to do so. For more details, see this document:
   http://dev.w3.org/html5/decision-policy/decision-policy.html

Status: Rejected
Change Description: no spec change
Rationale: Opera, Firefox and WebKit have treated HTML elements as being in the XHTML namespace as far as Selectors are concerned long before they implemented the HTML5 parser which put HTML elements in the XHTML namespace for real. So your example didn't work as you intended before either.