Scale a value from the root font size.
CSS: Rem units follow the document root instead of accumulating through nested font sizes.
<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>Example</title>
<style>
html { font-size:20px; } div { font-size:12px; } span { font-size:2rem; }
</style>
</head>
<body>
<div><span>Large label</span></div>
</body>
</html>Expected output EXAMPLE
At a 1000 × 800 CSS-pixel viewport, unless another size is stated: The “span” element has computed font-size “40px”.
Details
Save this complete fixture as example.html. The output describes layout or computed styles verified in Chromium. Rem units follow the document root instead of accumulating through nested font sizes.
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
08cae5a4d2cf1ecf406e69ced4cb31bbe33e5a65d60c18fbc51120401b3b0a84- 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.
Scale a value from the root font size: 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('span').evaluate('(e)=>getComputedStyle(e).fontSize')=='40px'