Web browsers use bloom filters for filtering out CSS rules during selector matching. Here's the implementation of this in Servo (the project I work on): https:

Web browsers use bloom filters for filtering out CSS rules during selector matching. Here's the implementation of this in Servo (the project I work on): https://github.com/servo/servo/pull/3212 https://github.com/servo/servo/pull/3212 I believe browsers also store their Safe Browsing (anti-malware/phishing) blacklists in bloom filters.