My understanding of MathML 3 and MathML Core is that <math> can contain more than one child element, and that by default, this should render the same as if there was a <mrow> between the <math> and the children.
But currently, the height of <math> elements (at least under horizontal-tb) seems to be affected by font-size in a way that an <mrow> child doesn’t (see attachment).
Also when <mrow> is omitted, we don’t seem to honour the writing-mode is always horizontal-tb rule properly, despite bug 1095933. Perhaps all bets are off until bug 1586574 and bug 1077525 are fixed, but I thought I would include this for completeness.
Assignee should remove this WPT workaround once they resolve the bug.