Check for actual MathML rendering

[Oblomov]

The HTML5 test checks that the browser correctly parses inline SVG and MathML, but does not actually verify that any attempt at actually rendering the content is made. Even if this is clearly annotated on the test, it would be appropriate to check that in addition to just parsing the tags, the user agent actually makes at least an effort at rendering.

For MathML, something like this can be used:

function hasMathMLSupport() {
        var div = document.createElement("div"), box;
        div.innerHTML = "<math><mspace height='23px' width='77px'/></math>";
        document.body.appendChild(div);
        box = div.firstChild.firstChild.getBoundingClientRect();
        document.body.removeChild(div);
        var has = Math.abs(box.height - 23) <= 1  && Math.abs(box.width - 77) <= 1;
        return has;
}

(adapted from here.)

[Oblomov]

A tentative patch implementing this is in PR #513