Support for non-JSX HTML (or at least good error message about issues with it)

[rufuspollock]

Should we allow support for traditional non-JSX html in markdown files?

Atm you can only use html if it is valid "jsx" so attributes must be camel-cased, class must be className etc. This is because we process all markdown files as MDX rather than markdown.

At very least i think we should have a good error message for this as i've seen this come up many times.

html style comments don't work

<!-- ... -->

Mention this because out of the box MDX (v2) does not support html comments and will throw and error like:

_mdx_bundler_entry_point-77f4c2f4-7bc6-4382-8a6e-cd2d3d4a7924.mdx:122:1: ERROR: [plugin: @mdx-js/esbuild] Unexpected character `!` (U+0021) before name, expected a character that can start a name, such as a letter, `, or `_` (note: to create a comment in MDX, use `{/* text */}`)

Instead mdx supports JSX comments e.g. {/* ... */}

See https://github.com/mdx-js/mdx/issues/1042 for discussion and suggestion of using a plugin like https://github.com/leebyron/remark-comment to resolve the problem