Quoting dabrahams in issue 1572710:

On second thought, I see what effbot is trying to say... but it's still a bug. Given the way the interface is declared and the behavior of regular python functions:

  Element(tag, attrib={}, **extra)

indicates that I can pass attrib (or tag, for that matter) as a keyword argument.  Nothing in the documentation gives the C implementation permission to behave differently.