I am in doubt. That wouldn't even compile. But who am I to think somebody changing something like this would actually do a test compilation afterwards....
HTML isn't compiled, and unknown attributes are allowed. The best practice is to prefix non-standard attributes with data- (e.g. <div data-foo="test">) but nothing enforces that. Custom attributes can be retrieved in JavaScript or targeted in CSS rules.
Yeah, you're right. My mind was stuck. No compilation for HTML of course. I was thinking of automatic testing in CI systems after you commit. Compilation for Java, C++, etc.; some other form of testing for HTML (renaming tags like these should throw some errors I suppose)