No need for floats!

After you've gotten float and clear working, try changing the sizes of the boxes and see what happens when they're too big to fit on one line. Then try flex-wrap & see how it works.

you can also arrange the boxes a little using the "order" and "flex-grow" attributes

try uncommenting some of the attributes of the "container" div. What do you notice?

Finally, put some content into the boxes and see what happens. Layout a little newsletter -- can you get it to look nice?