Math rendering doesn't work due to missing doctype

[kyllingstad]

I'm trying to view a local Markdown file (e.g. file:///path/to/file.md) with some math in it, but the math symbols don't render correctly. The dollar signs and the math code simply get included the rendered output. In the Firefox console, I see the following warning:

Warning: KaTeX doesn't work in quirks mode. Make sure your website has a suitable doctype. katex.js:17243:47