I think it can be theoretically done, like it is mentioned in the comment above, but you would need a new block of CSS for every slider min/max/step combination. It feels it would be harder to maintain than the javascript version.
Yeah I was thinking about something similar, e.g. using attr()* and a pseudoelement to pass the value from DOM to CSS of a ::before/::after pseudo-element.
But that still feels like asking for trouble:)
Luckily I need to leave the house now, I'm feeling CSS naughty.
* IIRC support for anything besides `content:` is still very unstable.
