I built a free responsive tester because DevTools only shows one device at a time

DevTools responsive mode has one limitation that's never been fixed: you see one device at a time.

You check iPhone. Looks fine. Switch to iPad. Fix padding. Switch back to iPhone. Was the header already broken or did you just break it?

I got tired of holding layouts in my head, so I built a tool that shows all three at once.

Responsive Tool — free, no signup

Paste a URL → phone, tablet, and desktop load at one time. That's it.

A few things that make it actually useful day to day:

• Swap one pane to a different device without losing the other two
• Refresh one pane after a code push — no need to reload everything
• Share your exact setup via URL — a teammate sees the same device comparison you do
• Sync scroll across all panes with a one-line script snippet
• Every viewport is a real verified CSS size, not a guess

No account. No extension. No download. Runs in the browser. I don't see or store the URLs you test.

Also: way more sites block iframes via X-Frame-Options than you'd think. I tested ~60 real sites — 93% of developer portfolios loaded fine, but only 44% of framework/marketing sites did. If your site doesn't load, that's the server blocking iframes, not a bug in the tool.

Stack

Next.js 16 · React 19 · TypeScript · Tailwind v4 · Cloudflare Workers

Bonus

I also built a CSS breakpoints reference that maps common breakpoints to real device viewports. Handy even without the tool.

👉 responsivetool.com

What do you use for responsive checking? Still just DevTools? I want to know what I'm up against.