A number of folks involved in WHATWG work bought into the XML vision initially, but reality has a strong text/html bias and we've been able to adjust views as experience has accimulated.
(Personally, the first time I managed to get funding to work on a Web engine was to make Gecko's XHTML-as-XML support better. At the time, I thought it was so important that I sought funding to get it done...)
See https://annevankesteren.nl/2011/02/xml-tired
(Personally, the first time I managed to get funding to work on a Web engine was to make Gecko's XHTML-as-XML support better. At the time, I thought it was so important that I sought funding to get it done...)