Preview renders unsupported HTML tags, but final post (correctly) doesn't

[cellio]

https://meta.codidact.com/posts/284503
https://meta.codidact.com/posts/284505

When composing a post, an unsupported HTML tag (div and kbd in these reports) was accepted and rendered. However, after submission the post doesn't render it, which is correct because we don't support the tag. This is confusing to users who haven't memorized (or don't look up) which HTML tags we do/don't support.

I know there's at least one other issue about differences between preview and final rendering, though I couldn't find it. We use different libraries in the two cases so differences aren't surprising. If we can't use the same library (or logic) to render the Markdown in both cases, is there anything we can do to provide some feedback when editing? Can we "lint" the post body and indicate if we found something? Maybe, as with missing alt text, that could be something we do when the user clicks the "post" button, so it doesn't have to be a performance drain.

[cellio]

Looks like it still renders in the preview: https://meta.codidact.com/posts/284505/286236#answer-286236