HTTPS DNS resource records can provide information about supported protocols such as HTTP/3 with "alpn" parameter. If Chrome receives a HTTPS DNS record indicating HTTP/3 support, Chrome will connect to the server using the HTTP/3 protocol.
Currently Chrome can use the HTTP/3 protocol only after receiving an Alt-Svc header from the server using a non-HTTP/3 protocol. With this new feature, Chrome can connect to the server using HTTP/3 protocol without receiving the Alt-Svc header. This can reduce 1 RTT of handshaking of TCP which is required for non-HTTP/3 protocol while connecting to the server.