The Content-Base is a standard entity-header field defined in RFC 2068.
It may be used to specify the base URI for resolving relative URLs within the entity.
This header field is described as Base in RFC 1808, which is expected to be revised.
If no Content-Base field is present, the base URI of an entity is defined either by its Content-Location (if that Content-Location URI is an absolute URI) or the URI used to initiate the request, in that order of precedence. Note, however, that the base URI of the contents within the entity-body may be redefined within that entity-body.