CSS Support Checker — Feature Detection
Generate a JavaScript snippet that tests 30 CSS features using CSS.supports() including Grid, Container Queries, and more.
How it works
- 1Configure your options above
- 2Click "Generate Detection Script" — processing happens in your browser
- 3Copy or download the result
What to do next
About CSS Support Checker
CSS Support Checker performs css support checker as a focused single-page utility. Generate a JavaScript snippet that tests 30 CSS features using CSS.supports() including Grid, Container Queries, and more. Defaults are tuned for the common case so the first run is one click, with every option that matters exposed for the moments you need to fine-tune the result.
From a technical standpoint, CSS Support Checker is JavaScript and standard browser APIs running in your tab. The browser is the runtime; the page is the interface. Maximum input size: 0 MB per run.
CSS Support Checker runs the entire transformation inside your browser. The file is read by JavaScript running in the page, processed in-memory by standard browser APIs, and written back as a download. The browser is the runtime; the page is the interface. You can confirm what the tool does by opening the developer-tools Network tab during a run — the only requests are for the page's own static assets.
If you fit any of these descriptions, CSS Support Checker should slot cleanly into your workflow: creators experimenting with formats; analysts pulling lightweight reports; site owners auditing pages. The tool keeps the controls focused on what matters for each of these use cases.
Reach for CSS Support Checker 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.
The hard constraints are easy to remember. Maximum input: 0 MB. Multiple files per run: no — one input at a time, by design, to keep results predictable. The same controls apply on every run.
For multi-step jobs, CSS Support Checker sits next to JS Support Checker, Browser Feature Detector, and Web Font Detector. None of them depend on each other — you can use CSS Support Checker on its own — but together they cover the common variations of the task this page exists to handle.
CSS Support Checker is built around steady iteration on a small set of options rather than feature creep. Every additional setting attracts a slightly different audience, but a long settings panel makes the common case slower for everyone. The current controls reflect what users of the tool actually use.
CSS Support Checker returns the result as a download. If you are running multiple jobs, the output names will not collide as long as the input names differ. You can re-run with different settings as many times as you like; each run produces a fresh file with no caching trickery in between.
A short note on how CSS Support Checker came to look the way it does: every iteration started by watching how someone unfamiliar with the tool actually used it, then removing whatever got in their way. That is why the upload area dominates the screen, the run button is bigger than the secondary controls, and the result panel is unmissable when the job finishes.
As a single-page tool, CSS Support Checker stays focused on one web and productivity utility step. Multi-step workflows are composed by chaining adjacent tools — each tool produces a standard file the next one can read directly, so a longer pipeline is just a sequence of short tab-and-tab visits.
Tips from users who reach for CSS Support Checker regularly: process one input first to confirm the settings produce what you expect before committing to a batch; treat the page as the working surface and avoid leaving large jobs running in a backgrounded tab where the browser may throttle JavaScript; and if a particular file fails, check whether the source is intact by opening it in its native viewer — most "tool errors" are actually input errors.
If the result is not what you expected, the most common causes are easy to check. Confirm the input is under the 0 MB ceiling — files just above the cap fail silently because the engine refuses to allocate the buffer. Confirm the input is one of the supported formats. And if the page itself feels slow, try closing other heavy tabs to free up memory; the engine runs in your browser, so it competes for the same resources as everything else open.
Open the workspace above to start using CSS Support Checker. The engine loads on the first interaction so the page itself stays light, and once the tool is warm it processes subsequent jobs quickly. The moment the page is interactive, the tool is ready to do real work on your file.
How it works
- 1Reach the CSS Support Checker page in your browser to begin.
- 2Add your web utility input by dropping it onto the page or browsing for it.
- 3Tweak the controls if the defaults are not quite right for your input. The options are kept short and labelled in plain language.
- 4Hit the run button. standard browser APIs does the work in your browser tab.
- 5Download the result. The file is generated in your browser and saved through your normal download flow.
- 6Repeat the process for additional inputs whenever you need to. The page stays loaded, so subsequent runs are quick.
Common use cases
- Generate a temporary asset for a social post using CSS Support Checker.
- Generate a campaign asset in seconds for a quick test.
- Validate a setting before circulating it to a team.
- Preview how a result looks before deploying it.
- Run a one-off check during a meeting without context-switching.
- Create a placeholder image for a wireframe.
- Audit a marketing page before launch.
- Plan content without paying for a SaaS dashboard.
FAQ
How many features?
Tests 30 CSS features from Grid and Flexbox to cutting-edge features like Anchor Positioning.
How does it work?
Uses CSS.supports() API which reliably checks if the browser understands a CSS property-value pair.
Cutting-edge features?
Includes Container Queries, :has(), color-mix(), oklch(), View Transitions, and Scroll-driven Animations.
Private?
Yes — runs locally.
Fallback strategies?
Use the results to decide which CSS features need fallbacks or polyfills in your project.
@supports in CSS?
The JS approach mirrors CSS @supports rules. Use @supports (property: value) for CSS-only detection.
Are there any restrictions on using CSS Support Checker at work?
CSS Support Checker can be used for personal and commercial work alike — there is no separate "business" licence to purchase. The output you generate is yours to use however you want, including in client deliverables, internal documents, or commercial products. Favtoo's only ask is fair, individual use; the tool is not designed to be embedded as a backend service or wrapped behind an API for resale.
Why is my browser prompting me when I open CSS Support Checker?
CSS Support Checker only needs the standard web platform — file picker access for the inputs you choose to load, and optionally clipboard access if you copy the result rather than downloading it. There is no microphone, camera, geolocation or background-permission request, because none of those are needed for the work the tool does.
Does CSS Support Checker support batch processing?
CSS Support Checker processes one input at a time by design — it keeps memory usage predictable on lower-end devices and makes results easier to verify. To handle a folder, run the tool once per file; the page stays loaded between runs and remembers your last-used settings, so the second run is essentially instant.
How often is CSS Support Checker updated?
CSS Support Checker 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.
Is CSS Support Checker lossless?
CSS Support Checker is built to preserve quality wherever the underlying web utility format allows it. Operations that are mathematically lossless (e.g. structural transformations, lossless re-encoding) round-trip with no perceptible change. Operations that involve a lossy codec inevitably introduce small artefacts at the byte level, but the defaults aim at the sweet spot where output looks or sounds the same to a normal viewer or listener while still being meaningfully smaller or faster than the input.
Why did CSS Support Checker reject my input?
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.
Can I self-host CSS Support Checker for my team?
CSS Support Checker is a static page running an open-source engine in your browser, so a typical corporate firewall does not get in the way as long as it allows JavaScript to load from Favtoo. For teams that need to host it themselves on an internal network, the underlying engine (standard browser APIs) is open-source and can be packaged into a private build with the same behaviour. Reach out via the Contact page if that is something you are exploring.
Is CSS Support Checker keyboard accessible?
CSS Support Checker uses native HTML controls wherever possible, which means keyboard navigation, focus rings, and screen-reader labels work the way the platform expects. The drop zone accepts files via the keyboard-accessible file picker as well as drag-and-drop, and result downloads use standard browser download flows. If you spot an accessibility gap, Favtoo treats it as a bug worth fixing.