IANAL, but as far as I know there’s no problem with distributing MIT software as a GPL component, since MIT allows imposing extra restrictions (like the share-the-source limitations of the GPL) to the code, so you can in theory turn every MIT software into GPL, what you can’t do is turn GPL software into MIT, so if the GPL software links MIT libraries that are part of its function, that instance of MIT software needs to follow the GPL.

Another thing is that this standard does not allow setting an absolute size… you can only use fractions, for example, 2/3 of normal size, which can still be relatively big if your normal size is big.

I also expect making things bigger for emphasis (or adding headings) would likely be more common than making them smaller (that’s what I hope at least). Outside of things like mathematical notation where superscript/subscript might be useful (see this comment for some examples).

The problem with the web is that some websites use absolute units that might not scale well (like px) to define the sizes.