Namespaces are not part of XML 1.0. They were invented about a year after the original XML specification was released. However, care was taken to ensure backward compatibility. Thus, an XML parser that does not know about namespaces should not have any trouble reading a document that uses namespaces. Colons are legal characters in XML element and attribute names. The parser will simply report that some of the names contain colons.
A namespace-aware parser does add a couple of checks to the
normal well-formedness checks that a parser performs. Specifically, it
checks to see that all prefixes are mapped to URIs. It will reject
documents that use unmapped prefixes (except for xml and xmlns when used as specified in the XML or
"Namespaces in XML" specifications). It will further reject any
element or attribute names that contain more than one colon.
Otherwise, it behaves almost exactly like a non-namespace-aware
parser. Other software that sits on top of the raw XML parser—an XSLT
engine, for example—may treat elements differently depending on which
namespace they belong to. However, the XML parser itself mostly
doesn't care as long as all well-formedness and namespace constraints
are met. Many parsers let you turn namespace processing on or off as
you see fit.