Found by headless e2e smoke pass. An element's background-color is painted only over the content box, not the padding box (which is the CSS default — background-clip: border-box).
Repro#
cargo run -p we-e2e -- --url crates/e2e/pages/06_flex.html --out /tmp/flex.png
See crates/e2e/artifacts/06_flex.png: each .item has padding: 16px; background: #cfe; border: 1px solid #6a6. The border outlines the padded box, but the green background fills only a smaller central rectangle — there is a white strip between the background and the border on all four sides.
Same defect on crates/e2e/artifacts/07_js_basic.png (the #out box).
Acceptance#
By default, an element's background-color paints from the inside of the border out to (and through) the padding area, matching background-clip: border-box. The smoke render of page 06 shows a contiguous green fill from border to border on each flex item.