Comment by ComputerGuru

Do browsers use a custom dictionary for zstd (I don’t think so since I can precompress zstd content server-side)?

Brotli was designed for html compression so despite/while being a relatively inferior algorithm, its stock dictionary is all html/css/js-trained/optimized. Chrome/Blink recently added support for seeing content compressed with a bespoke dictionary, but that only works for massive sites that have a heavily skewed new/returning visit ratio (because of the cost of shipping both the compressed content and the dictionary).

Long story short, I could see br being better than zstd for basic web purposes.