Exercise 1: A Parentless Element With an Empty Stylesheet — Possible Solution
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THE TEST
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    lonely = Element('p', {})
    lonely.parent = None
    empty_sheet = Stylesheet([])   # no rules at all

    compute_style_tree(lonely, empty_sheet)
    lonely.computed_style == INITIAL_VALUES

RESULT
------------------------------
    True -- lonely.computed_style is exactly equal to INITIAL_VALUES,
    property for property, value for value.

WHY THIS IS THE CORRECT RESULT
------------------------------
compute_style_tree calls compute_style(lonely, empty_sheet, None) --
parent_computed defaults to None since no parent_computed argument was
passed in for the root call. Inside compute_style:

    own = cascade(element, stylesheet)     # cascade() against an EMPTY
                                            # stylesheet has nothing to
                                            # loop over -- own == {}
    base = dict(INITIAL_VALUES)            # start from a COPY of the
                                            # defaults
    if parent_computed is not None:        # False here -- skipped
        ...
    base.update(own)                       # updating with an empty
                                            # dict changes nothing
    return base

Every branch that could have changed base away from a plain copy of
INITIAL_VALUES is either skipped (no parent to inherit from) or a
no-op (no own rules to apply). The function falls straight through to
returning an unmodified copy of the defaults.

WHY THIS WORKS AS AN ANSWER
------------------------------
This is the correct baseline case every other test in this chapter
implicitly depends on: an element with absolutely nothing influencing
it -- no ancestor, no matching rule -- has to fall all the way back to
CSS's own initial values, exactly as specified, with nothing else
mixed in. It's the "identity" case for compute_style(): remove every
possible source of influence, and what's left has to be precisely
INITIAL_VALUES, unchanged. If this case failed, it would mean either
cascade() was inventing declarations out of nowhere, or compute_style
was mutating INITIAL_VALUES itself rather than working from a fresh
copy of it -- a real risk, since dict(INITIAL_VALUES) has to make a
genuine copy each call, not reuse and quietly corrupt the same shared
dict across every element computed during a page's own style pass.