cssTricks.css

*{
  background-color: rgba(0,0,0,0.1);
}

Trick to debug when dealing with css box models.
by using the "*" you select all elements of the page.
Then, te next line of code is used to give every element a slighty darker color shade.
By doing this, every element which is nested in another element will get slightly darker than its parent element.

This is ideal to understand the padding and margins are affecting the surrounding elements.