Author here. I will admit that the labeling of the playground is a bit sloppy. The input comes before the output on desktop (as it should) but is flipped in mobile (for ... reasons), but I never wrote an addEventListiner on the matchMedia to change the labels to reflect the swapped order. As a front end engineer for 10 years this is pretty embarrassing.

Another missing feature which quite alot mobile users expect is jump to top. I personally never use it, but many mobile users do.

Code blocks also have kind of excessive margins, and I should probably shrink the font-size so that more users could read the entire thing without horizontal scrolling, though I think this is fairly minor.

Finally, what I can think of, is the content it self. I made the choice when writing the docs, to teach the reader a little about MathML as I demonstrate the project. I think this was a solid choice but it puts the simplest most unimpressive expressions at the top. On desktop this is fine because you can see more impressive expressions further down, but on mobile these are hidden in a horizontal scroll, so the user might be left unimpressed.