At the turn of the last decade, XML could do no wrong. There was no problem that couldn’t be solved by applying XML to it and every technology was going to be replaced by it. XML was going to kill HTML. XML was going to kill CORBA, EJB and DCOM as we moved to web services. XML was a floor wax and a dessert topping. Unfortunately, after over a decade it is clear that XML has not and is unlikely to ever be the dominant way we create markup for consumption by browsers or how applications on the Web communicate.

James Clark has XML vs the Web where he talks about this grim realization

Twitter and Foursquare recently removed XML support from their Web APIs, and now support only JSON. This prompted Norman Walsh to write an interesting post, in which he summarised his reaction as "Meh". I won't try to summarise his post; it's short and well-worth reading.

From one perspective, it's hard to disagree. If you're an XML wizard with a decade or two of experience with XML and SGML before that, if you're an expert user of the entire XML stack (eg XQuery, XSLT2, schemas), if most of your data involves mixed content, then JSON isn't going to be supplanting XML any time soon in your toolbox.
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There's a bigger point that I want to make here, and it's about the relationship between XML and the Web. When we started out doing XML, a big part of the vision was about bridging the gap from the SGML world (complex, sophisticated, partly academic, partly big enterprise) to the Web, about making the value that we saw in SGML accessible to a broader audience by cutting out all the cruft. In the beginning XML did succeed in this respect. But this vision seems to have been lost sight of over time to the point where there's a gulf between the XML community and the broader Web developer community; all the stuff that's been piled on top of XML, together with the huge advances in the Web world in HTML5, JSON and JavaScript, have combined to make XML be perceived as an overly complex, enterprisey technology, which doesn't bring any value to the average Web developer.

This is not a good thing for either community (and it's why part of my reaction to JSON is "Sigh"). XML misses out by not having the innovation, enthusiasm and traction that the Web developer community brings with it, and the Web developer community misses out by not being able to take advantage of the powerful and convenient technologies that have been built on top of XML over the last decade.

So what's the way forward? I think the Web community has spoken, and it's clear that what it wants is HTML5, JavaScript and JSON. XML isn't going away but I see it being less and less a Web technology; it won't be something that you send over the wire on the public Web, but just one of many technologies that are used on the server to manage and generate what you do send over the wire.

The fact that XML based technologies are no longer required tools in the repertoire of the Web developer isn’t news to anyone who follows web development trends. However it is interesting to look back and consider that there was once a time when the W3C and the broader web development community assumed this was going to be the case. The reasons for its failure on the Web are self evident in retrospect.

There have been many articles published about the failure of XML as a markup language over the past few years. My favorites being Sending XHTML as text/html Considered Harmful and HTML5, XHTML2, and the Future of the Web which do a good job of capturing all of the problems with using XML with its rules about draconian error handling on the web where ill-formed, hand authored markup and non-XML savvy tools rule the roost.

As for XML as the protocol for intercommunication between Web apps, the simplicity of JSON over the triumvirate of SOAP, WSDL and XML Schema is so obvious it is almost ridiculous to have to point it out.