Be glad that the browser authors are sticking to the RFC (1738). Chaos ensues when software makers don't follow RFCs, as those documents set the pace for standards on the Internet.
Using the port number instead of a service name is cross platform, as not all systems have consistent contents for /etc/services, or an equivalent.
Specifically in this case, section 3.1 of the RFC states:
3.1. Common Internet Scheme Syntax
While the syntax for the rest of
the URL may vary depending on the
particular scheme selected, URL
schemes that involve the direct use
of an IP-based protocol to a specified
host on the Internet use a common
syntax for the scheme-specific data:
The port number to connect to. Most schemes designate
protocols that have a default port number. Another port number
may optionally be supplied, in decimal, separated from the
host by a colon. If the port is omitted, the colon is as well.