mml3js xslt transforms failing on MS Edge [was: Readium on MS Edge: Unable to get property 'documentElement' ]

Via https://github.com/readium/readium-js-viewer/issues/394

On http://development.readium.divshot.io/?epub=epub_content%2Flinear-algebra with Edge, MathJax throws

Error: Unable to get property 'documentElement' of undefined or null reference

Probably caused by mml3.js, i.e., https://github.com/mathjax/MathJax/blob/master/unpacked/extensions/MathML/mml3.js#L92-L107 -- that ActiveXObject seems dubious (Edge kicked those kinds of things out, no?)