I deployed CSS changes on Magento but the old styles keep serving. Why?

The new CSS compiled, but a cache is serving the old file. On Magento that is usually a reused var/view_preprocessed directory, a stale pub/static/frontend theme folder, or a CDN that cached the old URL. The fix: wipe pub/static/frontend/<theme> and the matching var/view_preprocessed first, run setup:static-content:deploy -f so it does not reuse the cache, then version-bust the reference if a CDN is in front. We do all of this and verify the new bytes are served.