Short answer: Responsively App is a free, open-source desktop app (Mac, Windows, Linux) that shows a page on 30+ device sizes at once with mirrored clicks and scrolling, great for building layouts on your own machine. MobileCheck is a free online check that renders up to 51 devices, iPhones in Safari's engine, and lists readability problems with fixes. Build in one, verify with the other.
If you are looking for a free "responsive viewer" — one window that shows your site at many screen sizes — Responsively App is one of the best-known options, and it is actively maintained (version 1.18 came out in February 2026). It and MobileCheck are both free, but they work differently: Responsively App is a desktop browser you use while you build, MobileCheck is an online check that inspects a published page and tells you what to fix.
What Responsively App does
Responsively App is a free, open-source (AGPL-3.0) desktop app for Mac, Windows and Linux, described by its makers as a modified browser built on Electron. Its main features:
- All devices side by side in one window, from 30+ built-in device profiles, plus your own custom sizes.
- Mirrored interactions: a click, scroll or navigation in one preview is repeated in all of them in real time.
- One inspector for every preview, and one-click full-page screenshots of all devices.
- Hot reloading, so the previews update as you edit, and it can load your local dev server.
- Browser extensions to send a link from your browser to the app, and a built-in MCP server that lets AI coding agents drive it.
Because it runs on your computer, it can open localhost and pages you are logged in to. Electron embeds Chromium, so every preview — including the iPhone sizes — is drawn by Chrome's engine.
What MobileCheck does
MobileCheck runs in the browser with nothing to install. Paste a public address, pick any or all of 51 device profiles, and get a report in about a minute. iPhones and iPads are rendered in WebKit, Safari's engine; Android devices in Chromium, each with its real viewport, pixel ratio, user agent and browser bars. Every device is checked in normal view, dark mode, 150% text and with the mobile menu open.
Instead of leaving you to spot problems, it measures them: text that is too small, WCAG AA contrast, small or crowded tap targets, content wider than the screen, a missing viewport tag, disabled zoom, clipped or overlapping text, plus lab LCP, CLS and page weight. Each is marked on a full-page screenshot with a fix, and the report link can be shared. These are emulated devices, not real phones, and checks wait in one shared line.
At a glance
| Responsively App | MobileCheck | |
|---|---|---|
| Type | Desktop app (Mac, Windows, Linux) | Website, nothing to install |
| Cost and license | Free, open source (AGPL-3.0) | Free, no account |
| Devices | 30+ built-in profiles, plus custom sizes | 51 phone and tablet profiles |
| Browser engine | Chromium for every preview | WebKit for iPhones and iPads, Chromium for Android |
| Interaction | Live, mirrored across devices | Automated; the report is static |
| Finds problems for you | No, you look for them | Yes, with a score and fixes |
| Dark mode, 150% text, open menu | Not checked automatically | Checked on every device |
| Localhost and logged-in pages | Yes | Public addresses only |
| Hot reload while coding | Yes | No, re-run the check |
When Responsively App is the better choice
- You are writing the CSS right now. Hot reload plus a wall of screen sizes is the fastest feedback loop for breakpoints.
- The page isn't public. Localhost, staging behind a VPN and logged-in screens all work, because it runs on your machine.
- You need to click through a flow — a menu, a form, a checkout — and see every size react at once.
- You want an open-source tool you can inspect, extend or script, including from an AI coding agent.
When MobileCheck is the better fit
- You want to know what is wrong, not just see it. Eyeballing twenty previews won't tell you a link is 18px tall or that grey text fails contrast; the report measures every element.
- Your visitors use iPhones. A Chromium preview at iPhone size can hide Safari-only bugs; MobileCheck renders iPhones in WebKit and can compare both engines side by side on the Safari vs Chrome page.
- You need to check settings, not just sizes: dark mode and enlarged text change layouts as much as screen width does.
- You can't install software, or you want to send a client a link to the results.
How to use both together
- Build and tweak the layout in Responsively App on your local server.
- Once the page is on a public or staging address, run MobileCheck across the device range to catch what is hard to read or tap, and how iPhones render it.
- Fix the flagged issues back in Responsively App, then re-run the check. For things that come from iOS itself, such as the keyboard and system font, finish on a real phone.
Both tools emulate screens rather than run on phones, so neither replaces a final look on a real iPhone and a real Android phone before an important launch. Used together, though, they get you most of the way there for free.
Frequently asked questions
Is Responsively App free?
Yes. It is free and open source under the AGPL-3.0 license, with downloads for Mac, Windows and Linux.
Does Responsively App show how my site looks in Safari?
Not exactly. It is built on Electron, which embeds Chromium, so iPhone-sized previews are drawn by Chrome's engine. MobileCheck renders iPhones in WebKit, the engine behind Safari.
Can MobileCheck test localhost?
Not on the website: it can only load public addresses. For local work, a desktop tool like Responsively App or your browser's DevTools is the better fit.