Client side? i think not. 25 years ago we were told web sites were going to make their data available in nice machine readable XML form which would be transformed by xslt etc into presentation form and available for machine use without the presentation form. Same promise as semantic HTML but earlier, and same promise as webmcp now.
How is it failed? Just compared to, like, the prevalence of HTML?
I've worked in web dev for almost 20 years. Almost every year has had some kind of work with XML.
Client side? i think not. 25 years ago we were told web sites were going to make their data available in nice machine readable XML form which would be transformed by xslt etc into presentation form and available for machine use without the presentation form. Same promise as semantic HTML but earlier, and same promise as webmcp now.
We are using HTML and not XHTML. I have not used XML on websites in over 15 years when HTML5 got stable.