After a morning of searching, it seems there is no PHP Markdown library
that has built-in XSS/sanitization support. The recommended solution is
to use HTMLPurifier.
This actually works out OK, though, as it’s probably a good idea to
enforce sanitization regardless of which formatters are enabled, and to
not leave them with the responsibility of sanitization (it’s a big
responsibility). Since we cache rendered posts, the slow speed of
HTMLPurifier isn’t a concern.
Note that HTMLPurifier requires a file to be loaded by Composer, but
Studio does not yet support this, so for now I have included it
manually.