UI Checker JavaScript / CSS tool

I have a task (for instance): create an HTML&CSS according to the given Jpg file. Basically, what i want, is a simple JavaScript 开发者_运维问答lib (or whatever else it might be) which will allow me to see the transparent gif of the original file (50% opacity) and my actual page, so that i could fit all my classes pixel-to-pixel according to Jpeg.

Does it exist?

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This is a good extension, for Firefox:

http://www.pixelperfectplugin.com/