This looks good so far. When I was trying to build on this previously I got caught up on jinja within html tags, things like

<a href="/whatever">Whatever</a><input type="checkbox" name="mycheck" {% if mycheck_selected %}checked{% endif %}/>

That really spidered out places that needed to expect a jinja statement start. You've got a test that should cover the first case, what about the second?

If someone does something odd like

<{{ lol_tags }}>Things</{{ lol_tags }}>

Is it easy enough to make the error message "Invalid cratejoy syntax at line: " I can't think of a legit reason for that one, but it'd be valid jinja and potentially a pain in the ass to interpret.

They parse fine, although the jinja in that class isn't actually parsed. My goal isn't really to build the world's best parser but just to catch additional types of errors.

I've tested this against all of our merchants themes and it works fine / catches the things I want it to catch. It definitely can miss things, but I'm open to it being improved over time.