Browser interpretations of HTML are a indeed a hot mess. However, HTTP and HTML tend to grow together. By assigning new ports to HTTP/2 and HTTP/3 (and the ve

Browser interpretations of HTML are a indeed a hot mess. However, HTTP and HTML tend to grow together. By assigning new ports to HTTP/2 and HTTP/3 (and the versions of HTML that are prevalent at the time the protocols are released), older browsers that are not receiving updates would be completely unaware of the new ports and would only connect to the ports used for older versions where the clients would happily exist in their "I'm going to interpret things my way" world. When connecting to the new ports, browsers would have to agree to abide by the new well-defined and strict specifications. As support for HTTP/1 (and the associated older versions of HTML) disappeared, the HTTP/1 servers would either be shut down or configured to server redirects to the HTTP/2 and HTTP/3 content. In the latter case, should the client continue to issue HTTP/1 request to HTTP/2 servers, the server would send a "Version not supported" message telling the client to upgrade.