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Haha. 30 seconds after posting this comment I had a lightbulb moment (because I clicked the 'code' button here on UserEcho. I tried using


(i.e. escaped html) and that works just great!
Looks like you can't start with a "<" symbol either. I had a snippet to insert inline html comments so that I typed


and I received 

<!-- $0 -->

But when making a snippet with a tabTrigger of <! I get an error

Error parsing snippet xml: Error reading end tag. In file Packages/User/comment.sublime-snippet

(It's the same for either < and <! so it's clearly the < character it dislikes)