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One Click Test - Tool for quickly testing your design layout in one click.
UserTesting.com - Usability testing has never been easier. Get videos of real people speaking their thoughts as they use websites, mobile apps, prototypes and more!
W3C Mobile Checker - Make websites work better on mobile devices
Smartlook - Qualitative analytics for websites and mobile apps Start understanding the 'whys' of your users' behaviors with clear, visual insights. With session recordings and event tracking, you get the complete picture.
Flexbox Grid - Grid based on CSS3 flexbox