effbot: Nice, but your attributes need to be double-quoted instead of single-quoted. How about:

def mktag(tag, data, **attrs):
    tag = tag.lower()
    attrs = "".join([' %s="%s"' % (k.lower(), v) for k, v in
attrs.items()])
    if data:
        return "<%s%s>%s</%s>" % (tag, attrs,
data, tag)
    else:
        return "<%s%s />" % (tag, attrs)

 for tag in ("a", "ul", "li", "img"): # etc
    exec "%s = lambda d=None, **a: mktag(%r, d, **a)" %
(tag, tag)

 if __name__ == '__main__':
    print a("Google", href="http://www.google.com")
    print ul(li("item1") + li("item2") + li("item3"))
    print img(src="foo.jpg")

More XHTML-compliant (lowercase tags, empty tags) Too bad there's no easy way to HTML-escape data strings, but not the tags that are generated.