Center a fixed-size absolute child with auto margins.
CSS: An all-zero inset plus auto margins distributes the available space around the child.
<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>Example</title>
<style>
.parent { position:relative; width:200px; height:100px; } .child { position:absolute; inset:0; margin:auto; width:80px; height:20px; }
</style>
</head>
<body>
<div class="parent"><div class="child">Centered</div></div>
</body>
</html>Expected output EXAMPLE
At a 1000 × 800 CSS-pixel viewport, unless another size is stated: The child is centered inside its parent, with a top-left offset of 60 pixels horizontally and 40 pixels vertically.
Details
Save this complete fixture as example.html. The output describes layout or computed styles verified in Chromium. An all-zero inset plus auto margins distributes the available space around the child.
Tested on Ubuntu 24.04.5 LTS · · 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
7ed912f9aca67d89fc04b028704117141437b9c43d5e4dc09e6b232925d24381- 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 a fixed-size absolute child with auto margins: 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.
a=await page.locator('.parent').bounding_box(); b=await page.locator('.child').bounding_box()
assert b['x']==a['x']+60 and b['y']==a['y']+40