One aspect of host name syntax is hereby changed: the restriction on the first character is relaxed to allow either a letter or a digit. Host software MUST support this more liberal syntax.
The same document defines `in-addr.arpa` domains that have numeric labels.
The mandate of a starting letter was for backwards compatibility, and mentions it in light of keeping names compatible with email servers and HOSTS files it was replacing.
Taking a numeric label risks incompatibility with antiquated systems, but I doubt it will effect any modern browser.