That’s basically what happened with MathML a decade ago. Chromium had briefly had an implementation (M24 alone!), Firefox had an excellent implementation, and Safari had a decent implementation; there was very little usage. Chromium proposed removing it. There was much kerfuffle. When the dust settled, MathML won. MathML Core was defined, Igalia contributed a robust MathML implementation to Chromium, Safari tightened their implementation up, and people started actually using it a lot more.

Around the same time, Google tried to deprecate SMIL (SVG animation tech), which would probably have led to its removal after some time. This also failed, and it’s used more than it was then (though CSS animations are probably quite a bit more popular, and have over time become more capable), probably because now all engines support it robustly (MSHTML and EdgeHTML never did).

I hope that we’ll get a more formal commitment to XML and XSLT, and XSLT 3 in browsers, out of this.

As an aside, I note that MSHTML supported “HTML+TIME” (anyone remember Vizact?).