I want to have an XML attribute without any value, which simply has one meaning when it exists or does not exist.
Is that valid?
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.