While I think the MathML-for-CSS specification is useful, and I am moderately optimistic about the long-term prospects for using CSS to provide cross-platform rendering of MathML, I think that the default CSS sheet found as a non-normative appendix (§12) in the specification is in need of revision.
A copy of the default CSS sheet is mathml4css.css.
An example of a simple XHTML+MathML file linked with the default CSS sheet is binpow4css.xhtml.
Essentially the same content served as the text/html serialization of HTML5 and linked to MathJax — which should work in current major browsers — is binpow5.html.
Essentially the same content in an anonymous (from the viewpoint of a browser) source-level XML document type linked to a developmental CSS sheet geared to that document type is binpow.xml. (This should "work" in major browsers that support CSS-based rendering of XML documents.)
My first observation is that, as I think everyone anticipated, the default stylesheet interferes with the rendering of MathML provided natively by Firefox.
Secondly, there are at least two different major browsers in which the default stylesheet does not render the simple MathML in binpow4css.xhtml correctly as shown in the following screenshot of the two browser windows side-by-side: