Test Retina without a Retina Screen

I was sick of borrowing a friend's retina laptop to test retina images. After a little poking around, I discovered a few options baked right into the most popular browsers.

In Google Chrome:<br>
- Browse to chrome://flags<br>
- Search for and enable Force high DPI mode<br>
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In FireFox:<br>
- Browse to about:config<br>
- Search for layout.css.devPixelsPerPx