XML basically came out of SGML which had its origins in GML, which dates back to the 1960s. With XML they sort of dropped all the hard/difficult bits and used i

XML basically came out of SGML which had its origins in GML, which dates back to the 1960s. With XML they sort of dropped all the hard/difficult bits and used it as a simpler serialization format for tree shaped stuff. That happened after people got annoyed with another vaguely SGML inspired thing called HTML which of course was a bit lacking in specification and uniformity at the time. Which is why this whole thing came out of w3.org. So XML was specified and then right along came XHTML 1.0 and later 1.1, which was just whatever HTML 4 was supposed to be (Netscape, Opera, Microsoft, etc. had some conflicting points of view on this) but in well formed XML shape. Of course XHTML became a bit of a problem when it turned out that browsers had to deal with mostly not so well formed HTML coming from servers. People were just slapping whatever header on top of their html to trick browsers into various parsing modes. Eventually, the whole notion was dropped with HTML 5 just standardizing all the not so well formed bits in a way that browsers could make sense of. The WhatWG was responsible for that one. When it became clear that was the only HTML/CSS interpretation that still mattered, it was grandfathered into the w3.org and since then people have stopped talking about xhtml as a thing to pursue. Around the same time the w3c lost the plot a little bit and went off and built a whole sandcastle of standards on top of XML that mostly did not get a whole lot of traction. Capital S Semantic Web vs. lower case semantic web became a thing. One was a new web reimagined on things like SOAP, XHTML, RDF, and whole lot of other things. The other was web 2.0 featuring things like RSS (almost but not quite well formed xml, depending on where you got it), micro formats, hashtags, and a whole lot of people doing rather hacky things with wordpress, php, and related things. Then AJAX became a thing when MS added XmlRpcRequest. Soon, people figured out that this stuff just got a whole lot easier if you send Json over that instead of XML. Much easier to parse from Javascript for example.