Preventing “double” borders in CSS

Question

Say I have two divs next to each other (take https://chrome.google.com/webstore/category/home as reference) with a border.

Is there a way (preferably a CSS trick) to

Answer 1

I prefer to use another div behind them as background and delete all the borders. You need just to calculate the size of the background div and the position of the foreground divs.