Comment 4 for bug 179770

Yes indeed.

from http://codespeak.net/lxml/changes-1.3.5.html:

Bugs fixed 1.3.3
    * Element() did not raise an exception on tag names containing ':'

Other changes in 1.3.4
    * lxml now raises a TagNameWarning about tag names containing ':' instead of an Error as 1.3.3 did. The reason is that a number of projects currently misuse the previous lack of tag name validation to generate namespace prefixes without declaring namespaces. Apart from the danger of generating broken XML this way, it also breaks most of the namespace-aware tools in XML, including XPath, XSLT and validation. lxml 1.3.x will continue to support this bug with a Warning, while lxml 2.0 will be strict about well-formed tag names (not only regarding ':').