[xml] Is ignoring namespace and DTDs an xmllint or libxml2 problem?

[Thomas Gagne <tggagne gmail com>]

I'm unclear how I can reformat the file

tgagne ubuntu:~/tmp$ cat a.xml
<ns:a>
</ns:a>

without getting the errors

tgagne ubuntu:~/tmp$ xmllint --format --recover a.xml
a.xml:1: namespace error : Namespace prefix ns on a is not defined
<ns:a>
     ^

Is it a problem with xmllint or libxml2?