Stretchy characters not stretched in MathML-only document

[email protected]
Newsgroups gmane.comp.mozilla.devel.mathml
Message-ID <[email protected]> 
Hi,

Stretchy characters appear correctly in MathML embedded inside an
XHTML document, but if the same MathML content is read from a
non-XHTML file containing the math element as its root element, they
are not stretched. See and compare: 

http://intelsig.montefiore.ulg.ac.be/~piater/test/stretch.xhtml
http://intelsig.montefiore.ulg.ac.be/~piater/test/stretch.xml

Is there a reasonable explanation for this, or should I file a bug?
This happens in both Firefox 2 and 3.

Justus
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