Syntax Highlighting in Web Component Templates
https://remysharp.com/2025/11/12/syntax-highlighting-in-web-component-templates
The extremely quick read is, add /* HTML */ to the front of the template. Case sensitive and space sensitive (though hopefully one day it won't be so strict). Now highlighting and prettier (with save and fix) works.
Note that you need the es6-string-html VS Code extension for this to highlight correct (something I had forgotten I had installed).