问题
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?
回答1:
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.
回答2:
No.
Boolean attributes in XML are of the form foo="foo".
Even in SGML, you must provide the value, (it is the name, = and quotes that you can omit, which is why you have things like <select multiple> in HTML).
回答3:
Yes. You can have an attribute whose only permitted value is the empty string, "". I't not sure it's good design, though: I would normally suggest a boolean attribute with values true/false, and a default value of false.
来源：https://stackoverflow.com/questions/6926442/is-an-xml-attribute-without-a-value-valid