There does not seem to be any plan to add MathML support in IE soon. This is becoming critical now that MathPlayer is no longer supported in Internet Explorer 11 and that large websites like MDN are starting to use MathML content. Hence an alternative for the short term would be to implement MathML annotation / alttext so that Web developers can still provide alternate content for Internet Explorer. I already reported that for IE9 but the issue was fixed "by design" without further explanation so I hope this one will get more attention. At least support for semantics annotations and altext can be done very easily with CSS-only rules, as shown in the testcase I'll attach. So I hope Microsoft will consider integrating these rules in the IE user-agent stylesheet. I already reported a bug for Chromium and plan to submit a patch there.
For more information, see
1) A restricted version of the stylesheet landed in Chromium: https://src.chromium.org/viewvc/blink?revision=169439&view=revision
2) An alternative would be to use https://github.com/fred-wang/mathml.css to get support for a limited number of math constructions.