An attribute must be specified with the following syntax:
Name Eq AttValue
where Name is a legal XML name, Eq is = optionally preceded or followed by whitespace, and AttValue is a legal attribute value.
This definition is true for both XML 1.0 and XML 1.1.
If you are trying to specify an attribute as below:
<car owned/>
then no, that is not valid. If you are trying to specify it this way:
<car owned=""/>
then yes, that is valid.