a URI reference that resolves to a URL that will be used to resolve other URI references in the document.

Use percent escape codes as explained in the URL-encoding tutorial for any special characters in the URI reference.

If the value of the href attribute resolves to an HTTP URL, it may contain any of the following components:

1. protocol scheme, typically http: or else https:
2. username, followed by an "@"
3. host name or IP address
4. port number, which defaults to 80 for the http scheme and 443 for the https scheme
5. absolute or relative path
6. search query, indicated by "?"

If the protocol scheme, username, host name/IP address and port number are omitted the default is the current host - the same server as the base of the current document. If the path starts with a slash /..., it is an absolute path from the document root directory (AKA "web root") on the server. A relative path will be resolved relative to the base of the current document.