Exercise 2: An Invalid z-index Falls Back to 0 — Possible Solution
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THE TEST
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    box_with_auto = block({'z-index': 'auto'})
    get_z_index(box_with_auto)

RESULT
------------------------------
    0

No error is raised, despite 'auto' being a real, valid CSS keyword
that this course's own simplified z-index model never explicitly
handles as a special case.

WHICH PART OF THE FUNCTION IS RESPONSIBLE
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    def get_z_index(layout_box):
        style = box_style(layout_box)
        z = style.get('z-index', '0')
        try:
            return int(float(z))
        except (ValueError, TypeError):
            return 0

The try/except block is responsible. `float('auto')` is not a
mistake Python can recover from silently -- 'auto' is not a valid
numeric string in any sense, so float() raises ValueError:
could not convert string to float: 'auto'. Because that call sits
inside a try block, the except clause catches it (ValueError is
explicitly listed) and returns 0 instead of letting the exception
propagate out of get_z_index entirely.

WHY THIS IS A REASONABLE FALLBACK, NOT JUST A CRASH-AVOIDANCE HACK
------------------------------
Real CSS's own actual initial value for z-index IS the keyword
'auto' -- and 'auto' behaves, for stacking-order comparison purposes,
essentially like 0 (it participates in painting at the same "level"
as z-index:0 content, the key difference being whether it creates a
NEW stacking context at all, a distinction this chapter's own
scoped-down model doesn't attempt to implement). So falling back to
0 for 'auto' isn't merely "avoid crashing" -- it happens to land on
the right numeric behavior for sibling-order comparison purposes too,
even though the fallback path was written generically (to catch ANY
unparseable value, not specifically to special-case the string
'auto').

WHY THIS WORKS AS AN ANSWER
------------------------------
This is the same defensive-lookup pattern this course has used
repeatedly -- Chapter 6's DEFAULT_CHAR_WIDTH_EM fallback, Chapter 8's
DEFAULT_COLOR fallback -- catching an entire CLASS of unexpected input
(any string that isn't a clean integer-like number) with one generic
handler, rather than trying to enumerate every specific CSS keyword
that might appear in a z-index declaration (auto, inherit, initial,
unset, and so on all being real, valid CSS values this simplified
engine was never going to model individually).