Semicolon as URL query separator
The W3C Recommendation from 1999 is obsolete. The current status, according to the 2014 W3C Recommendation, is that semicolon is now illegal as a parameter separator:
To decode application/x-www-form-urlencoded payloads, the following algorithm should be used. [...] The output of this algorithm is a sorted list of name-value pairs. [...]
- Let strings be the result of strictly splitting the string payload on U+0026 AMPERSAND characters (&).
In other words, ?foo=bar;baz means the parameter foo will have the value bar;baz; whereas ?foo=bar;baz=sna should result in foo being bar;baz=sna (although technically illegal since the second = should be escaped to %3D).
As long as your HTTP server, and your server-side application, accept semicolons as separators, you should be good to go. I cannot see any drawbacks. As you said, the W3C spec is on your side:
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.