The Microdata spec (which is now merely W3C Note) defines:
If the element is an
a
,area
, orlink
element
The value is the absolute URL that results from resolving the value of the element'shref
attribute relative to the element at the time the attribute is set, or the empty string if there is no such attribute or if resolving it results in an error.
Exception: when the a
/area
/link
element has an itemscope
attribute, then the itemprop
"value is the item created by the element", not the href
value.