Center content with flexbox.

Center a child along both axes inside a CSS flex container.

<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>Example</title>
<style>
.box { display:flex; align-items:center; justify-content:center; width:300px; height:160px; }
</style>
</head>
<body>
<div class="box"><span>Centered</span></div>
</body>
</html>

Expected output EXAMPLE

At a 1000 × 800 CSS-pixel viewport, unless another size is stated:
The child is centered horizontally and vertically inside the flex container.

Details

Save this complete HTML example as example.html. The embedded CSS styles the fixture. Expected output describes the layout or computed style checked in Chromium.

✓ Verified

Tested on Ubuntu 24.04.5 LTS · 2026-11-02 18:01:07 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: c36c7a33ae132851cb1be8b406086b3bec033f5829e1b4450624e364d7f76ce6
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.

Center content with flexbox: 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.locator('.box').evaluate('(e) => getComputedStyle(e).display') == 'flex'
a=await page.locator('.box').bounding_box(); b=await page.locator('span').bounding_box()
assert abs(a['x']+a['width']/2-b['x']-b['width']/2)<1
assert abs(a['y']+a['height']/2-b['y']-b['height']/2)<1

# CSS # flex # center