Exercise 1: A Transparent Background Produces No Command at All — Possible Solution
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THE TEST
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    transparent_box = block({'background-color': 'transparent', 'width': '50px', 'height': '50px'})
    container = block({}, children=[transparent_box])
    layout_block(container, root_cb)
    build_display_list(container)

RESULT
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    []

An empty list -- not a rect command with color 'transparent', not a
placeholder of any kind. Genuinely zero commands.

THE EXACT LINE RESPONSIBLE
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    def paint_background(layout_box, display_list):
        style = box_style(layout_box)
        bg = style.get('background-color', 'transparent')
        if bg != 'transparent' and layout_box.dimensions is not None:
            rect = layout_box.dimensions.border_box()
            display_list.append(PaintCommand('rect', rect=rect, color=bg))

The line `if bg != 'transparent' and layout_box.dimensions is not None:`
is the guard responsible. Both boxes here -- the transparent child AND
the container itself, since block()'s own default style also leaves
background-color at INITIAL_VALUES' own 'transparent' -- have
bg == 'transparent'. The condition's first half, `bg != 'transparent'`,
evaluates to False for both, so `display_list.append(...)` is never
reached for either box. build_display_list() still walks the full
tree (container, then its child) via its own unconditional recursion,
but paint_background() itself contributes nothing at either level.

WHY THIS WORKS AS AN ANSWER
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This confirms 'transparent' is treated as a genuine, meaningful
sentinel value -- "there is nothing to paint here" -- rather than just
another color string that happens to render as see-through. A real
rasterizer benefits directly from this: a display list with fewer
commands is strictly less work to execute, and skipping transparent
backgrounds at LIST-BUILDING time (rather than emitting a command and
having the rasterizer itself decide to skip transparent pixels) means
the optimization happens once, up front, rather than being re-checked
for every pixel a transparent rect would have covered.