It is usually very hard to spoof TCP, which is the underlying protocol of HTTP (when ignoring QUIC, which has its own spoofing protection). But it is not impossible and it might be easier in some setups: see this link about quick blind TCP connection spoofing provided by ximaera for more information.
But, in case the attacker manages to successfully established a TCP connection with a spoofed source IP, the server will send its response to the claimed (spoofed) source of the connection. Since there is no matching association at the claimed target the packets sent by the server will either result in a RST from the spoofed target or will simply be lost. Since TCP relies on ACK from the peer (i.e. HTTP client in this case) to signal that it received some data and this ACK is not sent the server will realize that the connection is broken. And while QUIC is a different protocol than TCP the results will be similar.