The base element defines the document's base URI for resolving relative URIs contained within the document. A document cannot contain more than one base element. When present, the base element must appear in the head, prior to any elements that include a partial URI.
base's href attribute, required in XHTML 1.0 Strict, specifies the absolute URI used to resolve relative URIs. See Using Relative URLs for more details on using and resolving relative URIs.
Most Web pages do not require an explicit base URI since the document's URI is a suitable base. An explicit base URI is only required when the same document may be accessed at different URIs or when the document has no URI (e.g., sending an HTML document by e-mail).
The target attribute is used with frames to specify in which frame all links in the document should be rendered by default. The target frame specified by base can be overridden by a given link using the link's target attribute.
If no frame with the specified target name exists, the links are rendered in a new window unless overridden by the user. Special frame names begin with an underscore:
In XHTML 1.0, the target attribute value is case-insensitive, so that _top and _TOP both have the same meaning. However, most browsers treat the target attribute value as case-sensitive and do not recognize _TOP as having the special meaning of _top.