Exercise 1: A UA Rule With No Competing Author Rule at All — Possible Solution
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THE TEST
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    elem3 = Element('b', {})
    result3 = cascade_with_origin(elem3, UA_STYLESHEET, Stylesheet([]))
    result3['font-weight']

RESULT
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    'bold'

WHY THIS IS THE CORRECT RESULT
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cascade_with_origin loops over both origins in turn:

    for origin, sheet in ((0, ua_sheet), (1, author_sheet)):
        for rule in sheet.rules:
            ...

For origin=1 (the author sheet), sheet.rules is an empty list --
Stylesheet([]).rules == [] -- so that inner loop simply never executes
at all; nothing gets appended to matches from the author side. For
origin=0 (the UA sheet), UA_STYLESHEET's own 'b' rule matches elem3
directly, contributing (0, (0,0,1), <some order>, [Declaration('font-weight','bold')])
to matches.

After the loop, matches contains exactly ONE entry -- the UA rule.
Sorting a single-element list changes nothing, and the final result
dict is built purely from that one entry's own declarations:
font-weight becomes 'bold'.

WHY ORIGIN-AWARE CASCADING GIVES THE SAME ANSWER AS PLAIN CASCADING HERE
------------------------------
The whole POINT of checking origin before specificity is to resolve a
CONFLICT -- deciding which of two competing values, from two different
origins, should win. When only one origin has anything to say about a
given element and property at all, there is no conflict to resolve in
the first place: matches has only one candidate, and that candidate
wins by default, regardless of which origin it happens to belong to or
what sort key is used to rank it. Origin-aware cascading and plain
specificity-only cascading are mathematically guaranteed to agree in
every case where at most one rule matches -- they can only possibly
diverge once BOTH sides have a genuine, competing answer for the same
property on the same element, which is exactly the scenario the
chapter's own main margin example was built to demonstrate.