Substrate compatibility probe: a short line of ParitySans text at a known size on a baseline rule. It retains the raw Chromium comparison without treating Chromium's coordinate quantization as the sole geometry oracle.
Exact-output canary: two known, correctly stacked rectangles. Its current failure measures PDF coordinate serialization and painted-edge coverage after the shared pdftoppm path; it does not by itself prove incorrect block flow.
Exact-output canary: a known data:-URI PNG at a known CSS size. Its current failure measures PDF image placement encoding and painted-edge coverage after the shared pdftoppm path; it does not by itself prove incorrect CSS image sizing or decoding.
Exact-output canary: one known thick solid border. Its current failure measures PDF coordinate, stroke-state, and painted-edge encoding after the shared pdftoppm path; it does not by itself prove incorrect CSS border geometry.
