When you enable HTTP/3 on your CloudFront distribution, CloudFront automatically adds the Alt-Svc header, which it uses to advertise that HTTP/3 support is available and you don’t need to manually add the Alt-Svc header. We expect you to enable support for multiple protocols in your applications, such that if the application fails to establish a HTTP/3 connection it will fall back to HTTP /1.1 or HTTP/2. i.e., clients that do not support HTTP/3 will still be able to communicate with HTTP/3 enabled CloudFront distributions using HTTP/1.1 or HTTP/2. Fallback support is a required part of the HTTP/3 specification and is implemented by all major browsers that support HTTP/3.