I understood how it works more or less while I was playing around with it. I might have to really just sit down and figure out more how it works.
It seems to be working great. My only issue that remains is the last box not being flush with the bigger rectangle below.
When the browser window is at max width they are flush but as I start to shrink it(responsive), the narrower the window gets, the third block doesn’t stay align properly. They are a few pixels off.
Hmm.. I think it might be due to the 10px padding I have set on the bottom box.