Not sure exactly what you mean by how it works?
All web technologies essentially rely on the web browser for them to work. The web browser is like a VM running all your code. So the web browser needs to have a CSS parser that understands all the commands and can render them correctly according to the CSS spec.
The spec is constantly evolving (a “living spec”) and includes many drafts and proposals, plus individual web browser vendors like to innovate ahead of the spec, resulting in varied support for certain features among browsers, as well as vendor-specific features.
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