The MathML 3 spec says 'mstyle' defines a decimalpoint value (typically '.' or ','). That is used in 'maligngroup' and elementary math for decimal alignment. We have dropped it from core.

We have not made any decisions about mstyle in full, and perhaps decimalpoint can remain in it. However, I'm dubious about keeping about maligngroup and malignmark because AFAIK, only MathPlayer implemented those and no one uses it (it's way too complicated IMHO). That leaves the only use being elementary math and it is probably simpler (both in use and implementation) to add a decimalpoint attr to mstack than to support it in mstyle.

In writing this, I had thought that mtable supported decimal point as part of column alignment, but too my surprise it is only supported in 'maligngroup'. I don't remember why that omission. It is also not supported in CSS table alignment. It was maybe in HTML 4 but pulled from HTML 5 (see 'char' attr). There is a request to support something like in CSS from 2017, but that doesn't seem to be going anywhere. Maybe there is something else to address this in another CSS proposal?

Note: @davidcarlisle raised decimal point alignment in w3c/mathml-core#125, so it is slightly related to this issue.