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: 
An attribute must be specified with the following syntax:

> 
> Name Eq AttValue 
> 
> 
> 

where Name is a [legal XML name](http://www.w3.org/TR/xml/#NT-Name), Eq is [= optionally preceded or followed by whitespace](http://www.w3.org/TR/xml/#NT-Eq), and AttValue is a [legal attribute value](http://www.w3.org/TR/xml/#NT-AttValue).

This definition is true for both [XML 1.0](http://www.w3.org/TR/xml/#sec-starttags) and [XML 1.1](http://www.w3.org/TR/2006/REC-xml11-20060816/#sec-starttags). 

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.
You can have an attribute whose only permitted value is the empty string, "". I'm not sure it's good design, though; I would normally suggest a boolean attribute with values true/false, and a default value of false.