We just shipped support for the ugliest part of HTTP: Vary

Vary is the header everyone sets and no cache wants to honour. Cloudflare found almost 3,000 sites varying on four or more fields - some on 10, 23, even 47 - which gives you a cache that is technically correct and permanently cold. Their answer is three actions in Cache Rules: normalize equivalent headers, pass through the raw bytes, or bypass caching entirely. On all plans including Free. Worth reading just for the HTTP archaeology.