Prefixed attributes are in a namespace
Document creators may apply namespace prefixes to attributes to create 'global attributes' as are used in specifications like XLink.
Unprefixed attributes have no namespace
Unprefixed attributes do not fall into the default namespace. They have no explicit namespace, though they are generally assumed to be in the namespace of the element containing them.
It's not entirely clear whether a prefixed attribute that has the same namespace URI as its containing element is the same as an unprefixed attribute with the same local name.