CSS is both a language
and specification defining the typesetting (box) model. While there are certainly incompatibilities between different implementations, where CSS + HTML are well-defined, those would be bugs. That's why it's today actually possible to write one set of HTML and CSS and have it render identically in different browsers.
One could say the same for different TeX "formats" like LaTeX or Plain TeX which can be all be rendered with multiple mostly-compatible TeX engines. If you are familiar with TeX internals, it's outputting layouts of boxes for letters (TFM metrics files for fonts) inside pages (DVI files) that get filled later with bitmapped glyphs from bitmap font files (PK fonts). This is entirely different for PDF/PS-based TeX engines which use vector sizing and which are not identical to original TeX.
There are certainly more nuances (eg. hinting for vector fonts that aligns curves to pixel/dot grid), but languages and layout engines are usually tightly coupled.
