Insert missing keys using defaultdict

[Gnarlodious]

I am trying to grok this documentation but need help:
http://docs.python.org/library/collections.html#defaultdict-examples

In a perfect world the dict looks like this:
plistDict={'Style':'ExternalURL', 'Ref':'http://Gnarlodious.com/',
'Tip':'Opens in a new window', 'Text':'Gnarlodious.com'}

Let's say I want to prep a dict from a plist to insert the values into
an HTML link string:
"<a class='%(Style)s' href='%(Ref)s' title='%(Tip)s'>%(Text)s</a>" %
plistDict

However, in this imperfect world the dict might look like this:
plistDict={'Ref':'http://Gnarlodious.com/', 'Text':'Gnarlodious.com'}

which would error:
KeyError: 'Style'

So using defaultdict:
from collections import defaultdict

How do create a dict assigning every missing key with a default
string?

-- Gnarlie