Currently MathML doesn't define DOM/IDL things consistently with other spec areas of the platform, as such this leaves things under defined and MathML seems to be the one place that you can write markup and wind up with an Element (see
https://codepen.io/bkardell/pen/EMJqYz which reports Element as the constructor for me in FF, Chrome and Safari)

The downside of this is that it also winds up working considerably differently in important ways, making it harder to test, polyfill well, extend, or even just generally reason about. Effectively, it is currently 'outside the norm' of the platform currently.

To correct this and fit with the rest of the platform, we need to create some things. I've discussed briefly with some others and basically, the idea would be define a MathMLElement interface akin to SVGElement and then in HTML itself another like HTMLOrSVGElement which will probably have an unfortunately long name like HTMLOrSVGOrMathElement, but, first thing first - we should start with beginning to specify a MathMLElement and opening linked issues in HTML/CSSOM linking to those which give us access to things like .style, custom properties and so on and begin those conversations.