Exercise 2: A Fresh Centering Example With Different Numbers — Possible Solution
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THE TEST
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    f = block({'width': '250px', 'margin': '10px auto'})
    root_cb2 = Dimensions(Rect(0, 0, 400, 0), zero, zero, zero)   # 400px container
    calculate_block_width(f, root_cb2)
    f.dimensions.margin.left
    f.dimensions.margin.right
    f.dimensions.margin.top

RESULT
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    margin.left  -> 75.0
    margin.right -> 75.0
    margin.top   -> 10.0

CONFIRMING THE BOX IS GENUINELY CENTERED
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margin.left (75) + width (250) + margin.right (75) = 400 -- exactly the
containing block's own width. Both margins are equal, so the box has
identical empty space on its left and right -- the textbook definition
of horizontal centering. Working the arithmetic directly: 2-value
shorthand '10px auto' gives top=bottom=10, right=left='auto'. Both
horizontal margins are auto, so calculate_block_width takes the
`if ml_auto and mr_auto:` branch:

    total = 0(margin_left, zeroed) + 0(border) + 0(padding) + 250(width)
            + 0(padding) + 0(border) + 0(margin_right, zeroed) = 250
    underflow = 400 - 250 = 150
    margin_left = margin_right = underflow / 2 = 75.0

WHY THE VERTICAL COMPONENT IS COMPLETELY UNAFFECTED
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'10px auto' expands via expand_margin_shorthand's own 2-value rule:
`t = b = parts[0]` (10.0) and `r = l = parts[1]` ('auto') -- top and
bottom are set from the FIRST token, completely independently of
whatever happens to the second token (right/left). The auto-margin-
solving logic inside calculate_block_width only ever reads and writes
margin_left and margin_right -- it has no branch, no code path, and no
variable that touches raw_margin.top or raw_margin.bottom at all,
beyond the two unconditional lines near the end:

    top_margin = 0.0 if raw_margin.top == 'auto' else raw_margin.top
    bottom_margin = 0.0 if raw_margin.bottom == 'auto' else raw_margin.bottom

Since raw_margin.top is 10.0 (a real number, never the string 'auto'),
this line just passes it through unchanged: top_margin = 10.0.

WHY THIS WORKS AS AN ANSWER
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CSS's own constraint equation is defined purely in terms of
HORIZONTAL measurements (widths, horizontal padding/border/margin) --
vertical margin plays no role in solving for width at all, which this
engine's own code structure reflects directly: the entire auto-
resolution block operates exclusively on margin_left/margin_right,
never once referencing margin_top/margin_bottom in any of its
arithmetic. Testing with genuinely different numbers than the
chapter's own 300px/100px example (400px container, 250px width, and a
non-zero vertical margin mixed in) confirms the centering behavior and
the vertical-margin independence both generalize, rather than being
coincidences of the chapter's own specific numbers.