https://google.github.io/styleguide/htmlcssguide.html#Optional_Tags
Just to check though, am I reading this correctly that the recommendation in that Google style guide is to omit both the html opening and closing tags, not include the opening tag only? And btw that open tag is also what’s causing the indentation issues in your other issue filed here: Glitch keeps fighting my indentation - the editor sees the open tag, and therefore is trying to indent everything following it by one level.
That doesn’t change the confusing lack of error message of course and fact that we should consider making closing tags optional, but just wanted to point out the connection.