This chart looks like it could use some filtering for what constitutes a language used to build Firefox. It seems questionable that HTML is used to build 16 % of it. I suspect that is a result of test cases being included in the chart as it is based off of the whole repo. I checked out the repository and it doesn't have the GitHub language bar I see on other repos, so I can't click the HTML bit in it to filter down the HTML files in the repo and see if they are mostly tests or not, but it is hard to imagine they would be anything else. Maybe bits of the browser Chrome but still, that wouldn't be a whole 16 % I think.