Noob question.... I've occasionally seen that, while visiting some sites, my browser first sends out a

HTTP/2

request, which fails with a 404. The browser then sends a

HTTP/1.1

which succeeds with 200 and the site then loads as normal. Are there any specific response headers which influence

/2

vs

/1.1

choice?

HTTP/2 is not supported by most browsers on non TLS servers. Typically it is negotiated in the TLS ClientHello itself using ALPN. So you're probably looking at something else.

It might be the CDN configured to use HTTP2 wherever possible. Cloudflare has it, the user just has to do some clicky pointy things to enable it.