static/style.css is 51,425 lines in one file. Hundreds of selectors are
declared more than once and !important is used throughout, so the rendered
result is a function of source order. Extracting a block into its own file
changes that order, and nothing in the suite would notice - which makes a
51k-line split unfalsifiable and "looks fine to me" the only available
evidence.
This moves no CSS. It captures getComputedStyle over a fixed inventory of
676 elements across three pages, four viewports, both themes and the three
density modes - 16,224 element snapshots - hashes them, and compares against
tests/css_snapshot/baseline.json. A capture takes about 21 seconds.
The bench page synthesises one element per selector from an evidence-driven
list: every selector declared more than once in style.css that can be
expressed as a static compound chain, plus a curated set per feature area.
Redeclared selectors are the ones a reorder can flip. The bench loads
whatever stylesheets index.html ships, so it keeps measuring the real set
once the file is split. tests/test_css_computed_style_snapshot.py also carries
a self-test that swaps two conflicting .attach-strip declarations and asserts
the digest moves, so the harness cannot silently stop watching.
The second half is the asset-manifest check specs/frontend.md asks for,
scoped to stylesheets: every stylesheet referenced by shipped HTML and by the
sw.js precache exists, and index.html and sw.js agree on the ?v= string. They
hardcode it independently today, so a split that updates one and not the
other ships an offline cache nobody notices until a plane.
