In theory, yes; Github is working on / has deployed a web-based version of VS Code, and VS Code has a Markdown preview (and I'm sure there may be a WYSIWIG / Markdown editor as well).
It should be doable to create a WYSIWYG editor just for the MDN pages as well, or any static site generator for that matter. Question is whether they want to invest in that; will that actually improve things, given how the MDN pages are all fairly uniform in look, feel & layout.
That's yet another option, authoring Markdown with preview.
I've thought of GitHub REST API Pull Requests [1] this requires authorizing web application though.
Another version - is there any service that allows to post changes in Pull Request body? Maybe extend GitLab? Workflow would be Edit page, Preview, copy to clipboard, open new Pull Request, paste.
I am not a fun of Markdown anymore. WYSIWYG looks like much better approach, it already there. HTML to markdown tools are not great.
It should be doable to create a WYSIWYG editor just for the MDN pages as well, or any static site generator for that matter. Question is whether they want to invest in that; will that actually improve things, given how the MDN pages are all fairly uniform in look, feel & layout.