But more so, external style cannot be applied to a subsection of a web page unless they force it into an iframe, which has all sorts of issues of it's own which is why external CSS is usually ignored. Inline CSS is often stripped by the tag strippers who don't want you turning things on or off... and media queries shouldn't even play into it since the layout should be controlled by the page it's being shown inside (for webmail) or the client itself, NOT your mail.

If you want to implement a form with a superb User Experience, you have to take care of many variables:

One of the primary tasks of engineers is to minimize complexity. JSX changes such a fundamental part (syntax and semantics of the language) that the complexity bubbles up to everything it touches. Pretty much every pipeline tool I've had to work with has become far more complex than necessary because of JSX. It affects AST parsers, it affects linters, it affects code coverage, it affects build systems. That tons and tons of additional code that I now need to wade through and mentally parse and ignore whenever I need to debug or want to contribute to a library that adds JSX support.