It may be that I don't notice when I use it, if the page just translates itself into XHTML and I would never know until opening the developer tools (which I do often, fwiw: so many web forms are broken that I have a habit of opening F12, so I always still have my form entries in the network request log). Maybe it's much more widespread than I knew of. I have never come across it and my job is testing third-party websites for security issues, so we see a different product nearly every week (maybe those sites need less testing because they're not as commonly interactive? I may have a biased view of course)