If you want strictness, use a linter or a pretty-printer that follows your preferred style. Adopting an opinionated parser means you can't lint or pretty-print input from those with different opinions (I do not like underscores for emphasis), and thus somewhat goes against the goals of TFA here:

> Markdown files are essentially plaintext with some extra syntax for common elements like sections, bullet points, and links. The format deliberately avoids precise control over display details like font selection. Following the rule of least power, I consider this limitation a feature.

One of my biggest ongoing frustrations has been MDX - a sort of markdown-and-JSX mixture whose spec is now in its third release and which has made very little effort to maintain compatibility with either CommonMark or itself. It is fairly strict and fairly elegant, and moving to a new version requires rewriting all previously-written documents to eliminate no-longer-supported syntax and re-training writers. Both of those things are miserable tasks; it has absolutely killed any tolerance I might have had for a stricter parser.