Apply an enhancement only when a feature is supported.

CSS: A supports rule preserves the baseline outside browsers that understand the selected declaration.

<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>Example</title>
<style>
.layout { display:block; } @supports (display:grid) { .layout { display:grid; grid-template-columns:1fr 1fr; } }
</style>
</head>
<body>
<div class="layout"><span>A</span><span>B</span></div>
</body>
</html>

Expected output EXAMPLE

At a 1000 × 800 CSS-pixel viewport, unless another size is stated:
In a browser that supports CSS Grid, the layout uses display: grid. The feature query applies that layout only when supported.

Details

Save this complete fixture as example.html. The output describes layout or computed styles verified in Chromium. A supports rule preserves the baseline outside browsers that understand the selected declaration.

✓ Verified

Tested on Ubuntu 24.04.5 LTS · 2026-11-02 18:10:28 UTC · Chromium 141.0.7390.37 · libc6 2.39-0ubuntu8.9; Playwright 1.56.0

Test inputs, results and environment

The exact published code ran in a dedicated Linux test container with prepared inputs. Unprivileged snippet process, external networking disabled, read-only root filesystem, and a fresh temporary working directory. Fixtures and output records are isolated from the application database. Testing covers this example and its stated checks, not every input, operating system, or runtime version. The Linux container shares its host's kernel; this is not a native Windows test.

Architecture / host kernel: x86_64 / 6.18.54.1-microsoft-standard-WSL2
Snippet identity / locale: uid=65534, gid=65534 (unprivileged) / C.UTF-8
Interpreter invocation: Chromium: open example.html; execute the published browser assertions
Container image ID: sha256:32dc2d998158b6554c99b73b533f423696a0f3e31e8cbea9db10b5eee9a2fadd
Code SHA-256: e17ac5e7a0ec5cf706100951748ff4dd77b682bd38280052da7261fe6a9aae01
Dependencies: libc6 2.39-0ubuntu8.9; Playwright 1.56.0

Test input setup

These are the prepared inputs used by the test harness. See Details above for the prerequisites and how to run this example.

# No input files required.

Observed standard output

Expected output describes the rendered page. The standard-output record below is the browser test harness receipt; the additional assertions check the result and interactions.

Apply an enhancement only when a feature is supported: browser checks passed.

Exit code: 0. Standard error: empty. Validation: Shape/content assertions. System-dependent values above are one recorded run.

Additional assertions that passed

Checks recorded by the test harness. Where Python assertions are shown, browser cases use Playwright and simulator cases may inspect pin transitions: result.stdout is captured output, work is the temporary directory, and Path is pathlib.Path. The harness also imports json and csv.

assert await page.evaluate('CSS.supports("display","grid")')
assert await page.locator('.layout').evaluate('(e)=>getComputedStyle(e).display')=='grid'