The CSS and specs are maintained by W3C and are widely implemented by browsers. Likewise for the WAI-ARIA, WCAG, MathML, and SVG specs.
The XML and related specs are implemented by various applications and libraries even if web browsers dislike these specs. -- They are used a lot in document publishing workflows that use formats like JATS, and are supported by various tools and libraries.
SVG is widely supported in vector graphics applications and rendering tools.
And WHATWG hasn't just co-opted W3C specs -- it's also co-opted encoding, URIs and others from places like the RFCs.
The XML and related specs are implemented by various applications and libraries even if web browsers dislike these specs. -- They are used a lot in document publishing workflows that use formats like JATS, and are supported by various tools and libraries.
SVG is widely supported in vector graphics applications and rendering tools.
And WHATWG hasn't just co-opted W3C specs -- it's also co-opted encoding, URIs and others from places like the RFCs.