The ForestWiki.com uses PyOdide to syntax check Python Code.
I have been teaching classes on Python in the browser, based on PyOdide and soon Transcrypt. So I built a Python editor in the browser using the AceEditor, but it did not have syntax checking. So now on every keystroke, I compile the code, and flag errors. You can play with the editor here.
If you like this website, please upvote my Awesome Python pull request.
I am looking for editors/curators to help with branches of the tree. Please send me an email if you are interested.