3.0.21 fixed a security vulnerability in Markdown links by bringing in

bleach and bleach-allowlist, the latter of which contained

pre-defined HTML tags and attributes that are allowed by Markdown

output.
The list wasn't comprehensive enough, and it broke tables, code block

styling, and impacted emojis.
We're no longer using bleach-allowlist, and instead are defining our

own list of tags and attributes we consider safe for our needs. Along

with this, we now have unit tests covering all the Markdown features

that are either standard or provided through extensions we enable,

helping ensure we don't regress in the future.

Unit tests passed.
Manually tested that all formerly-broken functionality has been restored.