The v2 Mistune is created to be extendable. With the plugins and directives system, developers can extend it easily.
CommonMark has paid too much attention to the insane cases. Their tests contain so many weird cases that normal people wouldn’t write. Instead, they missed some real problematic cases, take an example:
It turns out on https://spec.commonmark.org/dingus/:
<p><a href="/hello"><a href="https://lepture.com">https://lepture.com</a></a></p>
A link can not contain another link in HTML. This case is an usual case, unlike other cases, we may generate different HTML in different parsers, but we should not generate invalid HTML.
Mistune has tests for CommonMark, but when some insane tests failed, we ignore them.