Problem:
You’re using KaTeX on a website, but every time you try to render, you encounter the following error message:
Uncaught Error: KaTeX parse error: KaTeX doesn't work in quirks mode.
at new e (VM697 katex.min.js:1)
at Object.l [as render] (VM697 katex.min.js:1)
[...]Solution:
KaTeX requires a valid DOCTYPE declaration on your HTML page, so add this line at the top of your HTML file(s), above <html>
<!DOCTYPE html>
Thanks to @xymostech on GitHub for the original solution in the the KaTeX issue tracker: