v0.1.14414 Aug 2026·Rendering, JavaScript & JIT, DevTools

Exact tiling: percentage columns never leave a seam

Second entry in the determinism line, after the WebGL invariant: layout now guarantees, bit-for-bit, that percentage columns tile their row exactly. Divide a row into thirds, quarters, five 20-percent columns - in every mainstream browser those widths land on an internal sub-pixel grid whose sum rarely equals the container, so a hairline of background bleeds through between abutting blocks at some window widths, and flex thirds famously end a fraction of a pixel past the box. Floati now resolves the rounding so adjacent column edges are the same number to the last bit and the final column closes the row exactly, for flex rows and float columns both. The guarantee is enforced by gate invariants that assert bitwise equality across two-to-twelve columns at eight hostile window widths, plus a bit-reproducibility check that layout is a pure function of the document. The overall score holds at 57: this is an invariant the reference browser cannot match, not a coverage move, and the scorecard only moves on coverage.

Parity with the reference browser

74overall, as of v0.1.343

Web APIs / DOM surface42

Security architecture66

GPU / graphics acceleration68

DevTools / extensions72

Networking85

Media85

Accessibility85

Real-world site compatibility99

HTML parsing100

CSS parsing + selectors100

Layout: block / flex / grid / tables / writing-modes100

Paint + visual100

JavaScript language100

JavaScript performance (JIT)100

HTML parsing100

What changed 4 changes

JavaScript & JIT	Flex lines snap a rounding-scale residual on the SHRINK side as they long have on the grow side: five 20 percent columns in a 977px row each round up in 32-bit float and summed to 977.00006, a hair of overflow the shrink loop's tolerance ignored. A sub-half-pixel residual on a shrinkable line now goes to the last item, so the line fits its container exactly.
DevTools	A start-packed flex line that fills its container closes bit-exactly: the last item's stored width is derived from the container's end edge and nudged by units-in-the-last-place until x plus width equals the edge as the same 32-bit float - not within a tolerance, equal. The reference browser's LayoutUnit grid cannot express this; its own DevTools shows flex thirds of 100px ending at 100.008.
Rendering	A float whose width lands within rounding distance of exactly filling the space beside the existing floats takes exactly that space, so percentage float columns - the classic two-up and three-up card grids - close their row without a seam or a spurious wrap.
JavaScript & JIT	The guarantee lives in the gate, not in prose: core-layout/tests/exact_tiling.rs sweeps 2 to 12 columns across eight float-hostile widths (primes and real window sizes) asserting shared edges and exact closure as bit equality, and asserts layout is bit-reproducible across runs. The one blessed-render change this caused was inspected pixel by pixel: two edge antialias shades on a gradient trophy, from the line now closing exactly. Companion note: docs/layout-determinism.md.