I am very impressed by the speed, but the typographic quality is notoriously ugly. Does it need to be that way? Does it happen only for the interactive demo? The screenshots look a bit better.
KaTeX strives to match LaTeX's behavior so it may actual be an issue with the screen readability of LaTeX. We're currently discussing ways to improve this in https://github.com/KaTeX/KaTeX/issues/1848. If you're seeing other issues please file a bug report.
