Comment by dev0001

From the GitHub thread (https://github.com/whatwg/html/issues/11523#issuecomment-315...)

> I don't think there's a strong "Open Web" argument to be made here. XML data files being able to be reformatted into HTML is somewhat of an accident of history; we don't have similar functionality for any other data type, despite, for example, JSON files being vastly more common on the web than XML.

Ironically, we would have support for transforming JSON if XSLT in the browser had been kept up-to-date.

Interestingly, "XML data files being able to be reformatted into HTML" was a deliberate choice to support and encourage the separation of content and presentation. CSS was introduced for the same reason but to address a slightly different (but complimentary) aspect of the same ambition and it's flourished. Imagine how widely CSS would be being used if browser support was still stuck at CSS 1.0.