They may appear in any order. Attribute names are always case-insensitive.
Attribute values are generally case-insensitive.
Values can be either relative or absolute URLs. Either this attribute or the NAME attribute MUST be present in the A tag. Either this attribute or the HREF attribute MUST be present in the A tag. None of the popular browsers ever implemented this attribute and it has since disappeared from the HTML 3.2 recommendation. The set of applicable names is a function of the protocol scheme of the URL in the HREF attribute.