You're assuming HTML string, which might not always be correct. What do you do with Markdown?
You need to have a standard way to represent things like this, because otherwise it won't gain traction solely due to the immense complexity of implementing viewers.
Edit: On top of this, HTML might not necessarily be the best choice. In that case it'd just be browser wrappers galore.
everyone is already used to making their frontend, backend and database parse and store external things in key value pairs
this schema pre-detecting ship has sailed a long time ago