A really cool tool, and extremely well presented. At first I thought: 'well why not just resize the browser window?'. But the options for device screen size is

A really cool tool, and extremely well presented. At first I thought: 'well why not just resize the browser window?'. But the options for device screen size is what makes this useful. Great job. Just one point. The dark overlay that comes up when the mouse is over the preview is a bit distracting. Maybe you could make this more subtle? What's the motivation for making this tool? Any ideas to monetize?