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When I started learning CSS, it was all about playing with floats and hacking things around absolute and relative positioning to kind of achieve what you wanted to do. Today we got cool new tools to create layouts: Flexbox and Grid Layout. If you forget about IE9 and older, Flexbox is supported pretty much everywhere and can be used to create flexible and stretchable layouts. For the moment Grid Layout, it is not supported everywhere, but still quite promising if you are looking for a way to build complex and versatile responsive grids.
Both modules can be a little bit scary and tricky to master. Fortunately, some nice people built quite a few and even fun tools to help you learn and master those so that you will be ready when they will be supported everywhere.