Per https://www.w3.org/TR/2014/REC-html5-20141028/forms.html#url-encoded-form-data, step 8 of creating URL-encoded form data, where it tells you how to add a second or subsequent key/value pair is:
If this is not the first entry, append a single U+0026 AMPERSAND character (&) to result.
That is, semicolons are no longer an acceptable option.
Why does this matter? Because modern tools and APIs are now treating semicolons as just plain data rather than an alternative to ampersand, and this has real impact. For example, the https://developer.mozilla.org/en-US/docs/Web/API/URLSearchParams browser API and corresponding polyfill https://github.com/WebReflection/url-search-params would be a handy way to replace our custom openils.CGI code.
But given a URL of https://example.org/eg/opac/advanced?bool=and;qtype=keyword, asking for the qtype parameter returns null, and asking for the bool parameter returns more than you would expect:
var params = new URLSearchParams(location.search.slice(1));
params.get('qtype');
// null
params.get('bool');
// "and;qtype=keyword"
We could continue using openils.CGI and adding our own workarounds to modern tools, but it might be better to standardize on ampersands sooner rather than later.
Per http://
Separate the name=value pairs in CGI parameter query strings with semicolons rather than ampersands. [...] Semicolon-delimited query strings are always accepted, and will be emitted by self_url() and query_string(). newstyle_urls became the default in version 2.64."
It's possible to return the behaviour to ampersands using the -oldstyle_urls flag. To be investigated.