On the contrary - Markdown is essentially a superset of HTML, so unless you're using a renderer that strips it from the input, you can have the best of both worlds.
This property was super useful for a lightweight CMS I threw together a few years and which is still used by the original customer today. 99% of what they need to render is easily authored in Markdown, and this further helps ensure a commonality of style and device portability.
The original markdown parser supported html because it was basically just a preprocessor that added some syntactic sugar to html. The proposal here isn't just "what if browsers had a markdown preprocessor" (although I also think that would be questionable), but "what if browsers limited content down to only markdown, so that the web was all just clean, style-agnostic documents," and that clearly requires that markdown not support arbitrary html.
On the contrary - Markdown is essentially a superset of HTML, so unless you're using a renderer that strips it from the input, you can have the best of both worlds.
This property was super useful for a lightweight CMS I threw together a few years and which is still used by the original customer today. 99% of what they need to render is easily authored in Markdown, and this further helps ensure a commonality of style and device portability.