I have "tinymist.typstExtraArgs": ["--features", "html" ] in my config and it works. When previewing, I get the warning “elem was ignored during paged export” since preview is not based on HTML export, but I get autocomplete and not compilation errors.
Have you restarted VS Code after changing the config? Can you share more about your setup?
An alternative could be using Bullseye to prevent your document from using the real HTML functions while previewing.