Why does a url use ://?
kwantam: RFC 1738 section 2.1 specifies the colon between the scheme (e.g., "http") and the scheme-specific-part (e.g., //news.ycombinator.com).  The // is specified in section 3.1 as part of the "common internet scheme syntax."  Specifically, the // is intended to identify the scheme-specific-part as complying with the CISS.