I found this app a few years ago that projects the photoshop file for a web page into the browser at reduced opacity. I forgot what it is called and have been looking for it ever since. Does anyone know or use something like this?
It's basically a camera obscura for the browser. You upload the asset you want the final page design to look like and then it overlays it over what you are designing in the browser so that you can match things to the exact pixel.