Is an xml attribute without a value, valid?

Question

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?

Answer 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.

Answer 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).

Answer 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.