[specialist]

CSS did not, could not, solve the problem of disorganized, inefficient document production, typography, layout, style management, etc.

Meaning, the Mixpanel abuse of styles is an old problem.

What CSS has over most all DTP and word processing apps is that CSS is declarative and explicit and discoverable. With CSS, styling is source code, and can be managed as such.

Or not, like Mixpanel apparently has done.