I opened a page wikipedia about 'em' in typography. By curiosity, I used the google dev tool (inspect element) to have look at the page's html source code. I find the following code which is very strange to me but I guess this is something very useful for performance analysis. How is it generated and by whom?
The HTML comment is a feature designed to help users fix pages and templates which hit the parser limits. It's controlled by $wgEnableParserLimitReporting.