It turns out that Chrome on Windows XP can't access four accents from the web fonts used by MathJax. These are the combining arrow above (U+20D7), the combining dot above (U+0307), combining double acute accent (U+030B), and the combining long solidus overlay (U+0338). The other browsers on XP (or any other platform) have no trouble with these, so I don't think it is a problem with the font. Historically, Chrome on XP has had some problems with the web fonts with a few characters being inaccessible (a "T" in one font, a "d" and ">" in another, and several others).
I may be able to work around these four, but that likely won't be available until the next release of MathJax. A quick check suggests that these are the only
problems in the current version of Chrome on XP.
For now, you can try switching to SVG output (right-click on an equation to get the MathJax contextual menu, and use the "Math Settings" then the "Renderer" menu to select "SVG").