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To download Duo from the Mac App Store, you need a Mac with OS X 10.6.6 or later. Learn More.


By Electric Pulp

Open the Mac App Store to buy and download apps.


Duo is a dual-paned web browser built for Responsive Web Design (RWD) that lets you browse the web in two views: your standard ‘desktop’ view as well as a typical ‘mobile’ view. By seeing these two views at the same time, you’re able to quickly test and work through multiple screen experiences rather than considering them separately.

Whether you’re a designer or a developer, project manager or client, Duo gives you a new tool to make your Responsive Web Design project go more smoothly than ever.

Get the bookmarklet:

Duo gives you an adjustable mobile view using the left pane.

The right pane defaults to an adjustable tablet / desktop view.

Navigating in either pane updates the other. No need to reload layouts to view pages at different sizes.

Duo has everything you need to make sure your design is doing what it should. Debug code in either pane.

Duo includes tools for clearing your cache and getting rid of those pesky cookies via standard key commands.

• LiveReload SUPPORT
Duo now includes support for LiveReload.

Use the convenient bookmarklet in your other browsers to push a site over to Duo. Get the bookmarklet:


Duo is designed for Responsive Web Design, not for user-agent sniffing. Duo also only refreshes both views for GET requests. It's probably not a good fit for your POST refresh heavy project, although we are actively looking at ways to accommodate this.

You can email for support, or reach out to us on Twitter: @electricpulp. Thanks!

What's New in Version 1.3

LiveReload Support


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Customer Reviews

Application does as advertised. A great way to quickly test multiple screen sizes.

Love the fact this has live reload support now. Duo has become a super handy tool to quickly test how things will look on multiple screensizes. It has a built in inspector, like Safari’s dev tools (which are prettier than Chrome’s IMO, but not as extendable… but I love how Safari can display minified CSS as expanded).

Just a suggestion or wishlist item: I wish I could add it to my Alfred Workflow, not sure what would need to happen to add Duo to the "Open URL with” option. Maybe it is already doable and I am not accessing it correctly?

Great screening tool — not a replacement for real testing

I build and do QA on a lot of web sites and I love this thing. It’s not a replacement for a real testing tool like BrowserStack or Sauce Labs, but it’s a great screening tool for use during development before you run tests on real testing tools like that.

I was a little nervous when I bought it that it hasn’t been updated in a while, but it works fine in El Capitan. Supports full-screen mode.

I agree with another reviewer that it would be even better if you could manually set the user agent string. With a list of common pre-sets. That would make it five stars. It’s still useful without that.

There is a “resize pane” button with a drop-down selector that you can add with “customize toolbar”. That really should be included in the toolbar by default.

Not real mobile view, very limited options...

Just purchased and tried, here is what I found wrong:

1. The mobile view gives you a “frame” of your website, and not the “real view” of it on a real mobile device. In other words, on my mobile phone I can see my complete website page, reduced in size of course, fitting the full screen, whereas on this program I can see just a portion of the screen without size reduction which is not the real view you get on a mobile device.

2. There is no way to access any settings to target specific screen devices. I really expected that!

For me, this app is useless and deceptive. To test my website for repsonsive design, I am going to use a real device.

View in Mac App Store
  • $4.99
  • Category: Developer Tools
  • Updated:
  • Version: 1.3
  • Size: 1.7 MB
  • Language: English
  • Seller:

Compatibility: OS X 10.8 or later, 64-bit processor

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