linkgrp = Group of pointers or refs. Use linkgrp to provide multiple, alternative
or simultaneously available, pointer or reference targets. The rel attribute
is used to indicate the nature of the relationship between this element and its
siblings, e.g. other linkgrp, ptr, or ref elements:
The rel attribute is %link.rel, and is used to indicate the nature of the relationship between this element and its siblings, e.g. other linkgrp, ptr, or ref elements: