box sizing border
The first challenge, was something which I was thinking about a lot these days. The next one is actually a well known common task, but it is still difficult. In every project we have different types of layouts. Of course there are some patterns. The idea today is to implement them with pure CSS.
The mixins in SASS are really helpful. Here is something really short which I just start using. It's a mixin which I apply on the grid's container.