 URL scheme

A URL scheme is the leading component before :, selecting a family of interpretation and processing rules. For example, https is the scheme of https://example.org/a. Scheme syntax begins with an ASCII letter and then permits letters, digits, plus, hyphen and period. Comparison is case-insensitive, and standard serialization uses lowercase.^[ref]

The scheme does not imply that every URL names a remote server. Compare https://example.org/a with data:text/plain,hello: both have schemes, but the second example carries data in its own spelling rather than naming an HTTPS host. A URL parser must apply the appropriate rules rather than demand a host and port for every case.

The colon also separates a scheme from a familiar relative-path interpretation. The text a:b is parsed as a URL with scheme a, whereas ./a:b can be a Relative URL reference. Adding the initial dot and slash therefore changes the syntactic role of the colon-containing text.

Scheme-specific defaults matter too: omitting port 443 when serializing an HTTPS URL reflects that scheme's default, not a rule that all numerical ports are redundant.^[ref]