About a decade ago it was a pretty commonplace thing to happen. HTML 4/4.1 was kind of messy, and could have rendering issues. So going with an (x)HTML validat

About a decade ago it was a pretty commonplace thing to happen. HTML 4/4.1 was kind of messy, and could have rendering issues. So going with an (x)HTML validator was a common thing, as well as a marketable value proposal to clients. HTML 5 had much "saner" implementations, so validators fell by the wayside as they weren't as necessary for compatibility.