I googled it, and came up with:
http://www.w3schools.com/tags/ref_urlencode.asp
The origin on why '%20' is used as a space in URLs
URL encoding the space character: + or %20?
http://www.blooberry.com/indexdot/html/topics/urlencoding.htm
and it appears to be part of an RFC spec. but not part of any language spec.
Maybe it is JavaScript. I don't know.
Is this true?