Polypane is a cross-platform browser for responsive web design and development. See your website in multiple screen sizes at once and make it look great, everywhere.
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“Designers should confront themselves with the reality of different viewports as early and often as possible. Polypane encourages this behaviour, and makes it easy to do. It's easily one of the most useful tools for modern web designers and developers.”Stephen Hay Author of Responsive Design Workflow