I've thought about this a bit too, and instead of a straight escape hatch to html when any special formatting is necessary, it might make sense to bind css to particular elements like so:

##[color:red] Heading 2

or using code blocks with particular headers which render out as normal text with the styling applied, like this:

```styling: 'ruleset here...' Lorem Ipsum Dolor ```

this does require a lot of work to consider what might be ergonomic for a particular element (by element I mean ATX headings, blocks, etc.), though.