CSS Validator — Check Syntax & Errors
Validate CSS for syntax errors, missing semicolons, unmatched braces, and unknown properties.
How it works
- 1Paste or type your text in the input field
- 2Click "Process" — processing happens in your browser
- 3Copy the result or download as a text file
What to do next
About CSS Validator
CSS Validator is part of a collection of single-purpose developer utility tools. Validate CSS for syntax errors, missing semicolons, unmatched braces, and unknown properties. Each tool is intentionally narrow — it does one thing well rather than offering many overlapping features — which makes the common path predictable and the result easy to verify before you download or copy it.
The engine behind the page is standard browser APIs. It reads your file in-memory and writes the result back into the browser. For 0 MB and below the work usually completes in seconds; larger files mostly depend on how much spare RAM your device has.
CSS Validator sees the most use from data analysts wrangling JSON and devops engineers crafting one-liners, but the design is intentionally generic enough that you do not need a specialist background to get a good result. The defaults aim at the most common case so a first-time user can get the right output without changing any settings.
CSS Validator is structured so the question "where is my file processed?" has a single answer: in your browser tab. The engine, the controls, and the result panel are all on one page. Navigating away or closing the tab clears the page's memory the way it does for every other tab.
Reach for CSS Validator when you need a predictable result on a single file. The page works on the first visit, the controls are visible without a menu, and the output is delivered the moment the engine finishes.
Once the engine finishes, the output is offered as an immediate download. There is no preview gate, no email-wall, and no "register to download" intermediary — the file is yours the moment it is ready.
The only practical limit is the 0 MB per-file ceiling, which keeps the tool responsive across a wide range of devices. Run the tool ten times in a row, run it ten thousand times — it behaves the same way and produces the same quality of result.
Workflow tip: CSS Validator pairs well with CSS to SCSS Converter and CSS Variable Extractor. Other adjacent tools you may find useful are CSS Specificity Calculator and HTML Validator. Because every tool is a separate page, you can mix and match the steps that match your job. Bookmark the ones you reach for the most.
CSS Validator is honest about scope: it handles a single, well-defined developer utility step. Specialist edge-case work — uncommon formats, very large inputs, or pipelines that need scripting — is what dedicated desktop apps are for. This page handles the common case quickly.
CSS Validator is one example of a broader pattern: utility software increasingly works as single-page, client-side experiences. Every page in the catalog is shaped that way, which keeps each tool fast to load and easy to recommend in a single link.
CSS Validator fits the gap where opening a desktop app feels heavy and writing a script feels overkill. The page handles the common developer utility task with sensible defaults so a single visit usually completes the job; for highly specialised work, a dedicated desktop application can offer more knobs to turn.
If you want to get the most out of CSS Validator, three small habits help. Drag-and-drop is faster than the file picker once you get used to it. The keyboard shortcut for downloading the result is whatever your browser uses for "save link as," because the result is a normal download. And if you are working on a sensitive file, processing in an Incognito or Private window is a good extra layer — it leaves no trace in browser history when the tab closes.
When something goes wrong, the cause is usually one of three things: a malformed input, a browser that is out of memory, or a corporate proxy that is interfering with the page's static assets. The first two are easy to diagnose; the third typically requires asking your IT team to allow standard browser APIs to load.
That is essentially everything CSS Validator does and how it does it. Open the tool above, drop in your input, and the work happens in the page. If you find yourself reaching for it often, bookmark the page — it loads quickly on subsequent visits, and your most-recent settings are remembered for the rest of the session.
How it works
- 1Open CSS Validator in your browser. The page loads quickly and the tool is ready to use the moment it becomes interactive.
- 2Select the developer file you want to process — drag-and-drop and the file picker both work.
- 3Tweak the controls if the defaults are not quite right for your input. The options are kept short and labelled in plain language.
- 4Click to start the job. The engine (standard browser APIs) processes the input in the page; you can watch the progress indicator until it completes.
- 5Grab the output as soon as the run completes. You can also copy the result instead of downloading if the next tool in your workflow accepts pasted input.
- 6Re-run with different settings as often as you want. Each run produces a fresh output and the original file on disk is never modified.
Common use cases
- Decode a token to confirm its claims during a debugging session using CSS Validator.
- Compare two API responses to spot a regression.
- Hash a string for a quick reproducibility check.
- Format a noisy log line into something a teammate can read.
- Validate a config blob before pushing to staging.
- Inspect a regex against a test string before committing it.
- Encode binary content for transport in a JSON body.
- Convert between data formats while wiring up an integration.
FAQ
What does this CSS validator check?
It checks for unmatched braces, missing semicolons, and unknown CSS properties against a comprehensive list.
Does it validate vendor prefixes?
Properties starting with a dash (e.g. -webkit-) are skipped; only standard properties are validated.
Can I validate SCSS or LESS?
This tool is designed for plain CSS. Use the SCSS-to-CSS or LESS-to-CSS converter first.
Does it check for browser compatibility?
No — it only checks syntax, not whether a property is supported in a specific browser.
Does it handle media queries?
Yes, @-rules are recognized and brace matching works across nested blocks.
Is my data safe?
All processing happens in your browser.
Can I use CSS Validator on iOS or Android?
CSS Validator runs in any modern mobile browser — Safari, Chrome, Firefox and the in-app browsers in most messaging apps all support the underlying APIs. Performance depends on the device: a recent phone handles typical inputs nearly as fast as a laptop, while older devices may take a few seconds longer near the 0 MB ceiling. The interface lays out cleanly on small screens, so you do not need to pinch-zoom to see the controls.
Will CSS Validator ask me to pay to download the result?
CSS Validator is free to use. The processing runs in your browser, which keeps the per-user cost low enough that the tool can be offered openly. The download is the same file the engine produced — you can use it for as many runs as you need.
Will CSS Validator keep working in a year?
CSS Validator is updated whenever the underlying engine releases an improvement or a bug fix. Because the tool is delivered as a static page, every visit fetches the latest version automatically — there is no "version" to manage on your end. If a particular release ever changes default behaviour, the change is documented on Favtoo's changelog so you can confirm what shifted.
Will CSS Validator keep working if my Wi-Fi drops mid-task?
Once the page is loaded, CSS Validator can complete jobs without an active internet connection — the engine is bundled with the page, so there is no per-job network call. The initial page load does require a connection (to fetch the static assets), but after that you can disconnect entirely and the tool will still work. This is a side-effect of the local-first architecture, not a deliberate "offline mode" feature.
Is it safe to use CSS Validator on confidential files?
Your file is processed inside your browser by standard browser APIs. The engine reads the file's bytes from your tab's memory, computes the result, and writes the result back into the tab. You can confirm what the page does by opening developer tools and watching the Network tab during a run — the requests you see are for the tool's static assets only.
What does the error message in CSS Validator mean?
Failures usually fall into one of three buckets: the input is in an unsupported format, the input is over the size cap, or the input is structurally malformed (a truncated download, a partial export, or a stream the engine does not recognise). The first two are easy to confirm — check that your file is in a supported format and that it is below 0 MB. For the third, opening the file in its native viewer first is the fastest way to confirm the source is intact.
Why does CSS Validator feel slow on large inputs?
Most jobs finish in seconds. Speed scales with input size and with how many CPU cycles your browser tab has available — the engine runs in your browser, so it shares resources with whatever else you have open. For inputs near the 0 MB ceiling, expect anywhere from a few seconds to roughly a minute on a typical laptop. Closing other heavy tabs noticeably speeds things up.