Comment by TheRealPomax

Except under GFM, html is explicitly part of the spec[1], not just "it's not a parse error, but that's because it's just text that doesn't fall in any predefined markdown syntax category", and good luck finding a WYSIWYG markdown editor that gets them right. The number of editors that completely break on <details> alone is disheartening.

[1] https://github.github.com/gfm/