« B.2.2: Ampersands in URI attribute values
The URI that is constructed when a form is submitted may be used as an anchor-style link (e.g., the href attribute for the element). Unfortunately, the use of the "&" character to separate form fields interacts with its use in SGML attribute values to delimit character entity references. For example, to use the URI "http://host/?x=1&y=2" as a linking URI, it must be written <A href="http://host/?x=1&y=2"> or <A href="http://host/?x=1&y=2">.
We recommend that HTTP server implementors, and in particular, CGI implementors support the use of ";" in place of "&" to save authors the trouble of escaping "&" characters in this manner.»
It's not a new recommendation either. It's been there since 1995. CGI.pm supports it, but I doubt I've ever seen it actually used anywhere.