Best Practice for Vertical Math (Elementary)

I work with math accessibility, but primarily at the secondary and postsecondary levels. I have a client who has a lot of elementary math content. They want to adhere to MathML specifications for vertical math (for example, using mstack for stacked addition). The educators use MathType for input and are using the symbol it provides for vertical addition which outputs a MathML configuration using mstack, which is not supported, since it is outside of MathML core.

What is best practice right now, in 2026, for formulating elementary vertical math? The particular question I am trying to answer from an educator is not only a stacked addition problem, but one with decimals that must be aligned. Is an mtable based solution the right way to go? Is there another authoring pattern that is recommended instead?

I also am anticipating the question of what happens to this content if later on elementary math is added to core and mstack is supported? Should the legacy formulations then need to be redone so that the screen reader experience is the same for similar vertical math constructions that are coded different ways?

My practical concern is I've got non-technical math educators with MathType as their input tool (not using a LaTeX to MathML workflow which is what I'm used to) trying to create elementary math using the MathType tool, so we need to be able to tell them what will work best, that is also somewhat future-proof. It's hard to explain why MathType includes a tool that allows the exact formulation they need, that is also the formulation recommended by the MathML spec, that they must nevertheless avoid using because it is not supported by browsers.

I'll add here that another issue is that if the publishing environment were only web, I would advise trying the MathJax experimental MML3 elementary math extension to see if that would enable simple mstack formulations (no strike outs or carries, just the vertical math). I'm also evaluating workflows for MathML in PDF and there is EPUB 3 output to consider.

Any current thoughts on this pain point that is elementary math? As there so often is with specifications there is a gap between the ideal and what is actually supported and feasible.