What is it?
The CSS Checker finds the small mistakes that make a style silently do nothing. It reads your CSS with your own browser’s CSS engine and lists unknown properties (with a “did you mean” spelling), values the property does not accept, broken selectors, missing semicolons and braces, empty rules and properties set twice in the same rule.
Who is it for?
Web designers, bloggers customising a theme, and students who changed a colour or margin and saw nothing happen.
How to use it
- Paste your code into the CSS to check box, or tap Try a sample to see how it works.
- The list of problems updates as you type: ✕ marks an error, ! a warning. The line above it sums up, for example “2 errors, 1 warning. Tap Show to jump to the line.”
- Tap Show next to a problem to jump to that line, fix it, and watch the list update. Problems that are not on one line say Whole file. Clear empties the box.
Worked example
Paste a{colr:red}. The checker says the property is unknown and suggests “color”. Change it and the list clears. Next try a{color:redd}: now the property is fine but the value is not accepted.
What it understands
@media, @supports, @container, @layer, @keyframes and @font-face, CSS nesting with &, and custom properties with var(). Vendor-prefixed properties such as -webkit-line-clamp are skipped on purpose, because they are meant for other browsers. If a property is set twice in one rule, the later one wins, which is why a duplicate gets a warning.
Is my code uploaded?
No. The checker is downloaded once from a public download server (jsDelivr or cdnjs) and then runs inside your browser. Your code never leaves your device, so it is safe for work and client projects.
Errors and how to fix them
- “✓ No problems found: this browser understands every rule”: the CSS itself is fine. If the style still does not apply, another rule with a stronger selector may be winning; inspect the element in your browser.
- Unknown property … (did you mean …?): a spelling mistake such as
colr. Use the suggested name. - Invalid value: for example
color: reddormargin: 10without a unit. Check the spelling and add px, rem or %. - Missing semicolon: the browser also drops the next line, so fix these first.
- A brand-new property reported as unknown: your browser does not support it yet. That is useful to know before you rely on it.
Open the tool: CSS Checker
Still stuck?
Contact support. Tell us which tool or page you were using, what you tapped, what you saw, and your phone or computer and browser (for example: Android phone, Chrome). We reply by email.
Questions, or an error not listed here?
Tell us in the comments: what you were doing, the exact message you saw, and your device and browser. We read every comment and add new fixes to this article.