> The size of the circle on the contour integral is a matter of personal preference, but it just depends on the typeface and is orthogonal to the choice of LaTeX/MathML.

Indeed! That was precisely the point of my previous comment.

I agree that switching to Latin Modern resolves some of the minor issues I mentioned earlier. However, it does not resolve all of them. In particular, it does not address the spacing concerns I mentioned earlier. For example compare the following on <https://mk12.github.io/web-math-demo/> with Latin Modern selected:

  \sum_{q \le x/d}

Or:

  \sum_{d \le \sqrt{x}}

The difference in spacing is really small but it is noticeable enough to bother me. Also, this is just one of several examples where I wasn't happy with the spacing decisions in MathML rendering. The more time I spent with MathML, the more such minor annoyances I found. Since KaTeX produces the spacing and rendering quality I am happy with, out of the box, I have continued using it.

Also, my goal isn't to replicate LaTeX's spacing behaviour faithfully. I just want the rendered formulas to look good, close to what I find in print or LaTeX output, even if it's a bit different. It so happens that I find myself often bothered by some of the spacing decisions in edge cases when using MathML, so I tend to just stick with MathJax or KaTeX.

But that's just me. All of this may seem like nitpicking (and it certainly is) but when I'm spending my leisure time maintaining my personal website and blog or archiving my mathematics notes, I want the pages to look good to me first, while still looking good to others. If MathML output looks good to others with certain fonts, that's a perfectly valid reason to use it.