The W3C went down a rabbit hole of XML based stuff with an emphasis on everything being well formed and valid. There was a time 15 or so years ago when XHTML a

The W3C went down a rabbit hole of XML based stuff with an emphasis on everything being well formed and valid. There was a time 15 or so years ago when XHTML a version of HTML which was also valid XML was being touted as the way forward. This meant that if there were any errors in the page it would just though an error, as opposed to HTML which would try it's best. WHATWG was created in frustration with that approach and saw the XHTML and other XML stuff being a case of Architectural Astronomy (which is not to say they weren't doing anything, just they weren't doing anything seen as useful) and they created HTML5 which was a standard based on how HTML worked in the wild and explicitly codified how tag soup and other invalid pages should be shown. HTML5 really revitalized the web at a time when it was pretty stagnant. WHATWG has always had a pretty practical approach to standards that boils down to the fact that whoever builds a browser ends up having the final say in what goes into the spec because it doesn't matter if there is some cool feature that nobody wants to ship, if nobody ships it it's fundamentally not part of the spec because it can't be used. This does end up meaning that WHATWG ends up being just made up of Microsoft, Google, Apple, and Mozilla. There is some irony as the descriptive HTML5 which tried to codify how HTML worked in the wild made it so complicated to create an HTML parser that now only a megacorp could actually afford to do so.