Re: [w3ctag/design-reviews] MathML Core (#438)

> What is the meaning of "legacy compat" in this context? It doesn't seem to be called out in the spec.

Yeah, I guess this could have been clearer.  Basically, they are things which, in an ideal world, would just be solved by existing platform answers.  However, a significant amount of real world legacy content uses it, so we think the right thing to do is to not break that - to map to the existing and include it in the spec as deprecated. HTML has several similar things - `<font>` and `<center>` for example.

> How is this parsed? Do all the elements need to be explicitly closed?

This is all specified in the HTML Parser itself - which is complicated, but I think the special cases you are probably interested in will be listed here:
https://html.spec.whatwg.org/multipage/parsing.html#the-stack-of-open-elements

> Overall, there is some tension in the choice of core elements between whether the elements are expressing semantics or layout.... [snip] Is there any chance that this could be generalized to something like <mgroup>

There are probably lots of choices that would be made differently if we were starting from nothing today - I think the challenge here is that the aims are to normalize and well-define what is necessary to keep millions of existing mathml contents in play - `<mrow>` is pretty central to that.  We could potentially propose to _also_ have an `<mgroup>` but so far we've tried to not add new invention of that sort here for the initial definition of core.



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