HTML & XHTML: The Definitive Guide, 6th Edition

Referencing Documents: The URL

Every document on the Web has a unique address. (Imagine the chaos if they didn't.) The document's address is known as its uniform resource locator (URL).^[†]

Several HTML/XHTML tags include a URL attribute value, including hyperlinks, inline images, and forms. All use the same URL syntax to specify the location of a web resource, regardless of the type or content of that resource. That's why it's known as a uniform resource locator.

Because they can be used to represent almost any resource on the Internet, URLs come in a variety of flavors. All URLs, however, have the same top-level syntax:

scheme:scheme_specific_part

The scheme describes the kind of object the URL references; the scheme_specific_part is, well, the part that is peculiar to the specific scheme. The important thing to note is that the scheme is always separated from the scheme_specific_part by a colon, with no intervening spaces.

Writing a URL

Write URLs using the displayable characters in the US-ASCII character set. For example, surely you have heard what has become annoyingly common on the radio for an announced business web site: "h, t, t, p, colon, slash, slash, w, w, w, dot, blah-blah, dot, com." That's a simple URL, written:

http://www.blah-blah.com

If you need to use a character in a URL that is not part of this character set, you must encode the character using a special notation. The encoding notation replaces the desired character with three characters: a percent sign and two hexadecimal ...