The answer is unfortunately simple: MathML is not supported in Chrome, Opera and IE.
Firefox has a very solid implementation (but not yet complete), Safari only a very partial implementation. Neither Firefox nor Safari support Content MathML.
That's why you'll need to use a polyfill like MathJax (cf MathML support) or jqmath.
Neither of those polyfills supports Content MathML, so you'll want to follow MvG's advice to produce Presentation MathML.
On older IE (6-9), users can install the MathPlayer plugin which is the only implementation supporting all of Presentation and Content MathML.