Check every rule in code

How to do it

Write each rule as a build gate, a test or a script with an exit code. Zero means the rule holds, anything else means it is broken. The output names the rule, the file or page and the line, so nobody has to read, guess or interpret. A rule without a check is only a wish, so add the check in the same change as the rule.

Example

// A rule as code: pass or fail, never "looks fine"
const problems = pages.filter(p => (p.html.match(/<h1[\s>]/g) || []).length !== 1);
for (const p of problems) console.log(`FAIL ${p.path}: needs exactly one h1`);
process.exit(problems.length ? 1 : 0);

How to check

Break one rule on purpose. The check fails, names the place, and exits with a code other than zero.

Measured

On a reference site, the first full run of the checks found 678 sideways-scrolling failures. One CSS line in a new menu caused all of them, and the menu had looked fine in screenshots.