It took a little effort to get it just right, but I’ve been able add CSS quickfix and error reporting to the typescript-lit-html-plugin language plugin for VS Code. In the video below, we can see css quickfix and error reporting working for the
<style> block without any fancy user settings (albiet with a flame emoji that didn’t make it into the pull request :-). It’s also smart enough to understand the difference between the various other pieces of code that might be in your tagged template.
Many thanks again to Matt over at Microsoft for putting up with my questions and taking the time to review and merge that pull request.