You aren't specifying the "height" of your html. When you're assigning a percentage in an element (i.e. divs) the css compiler needs to know the size of the parent element. If you don't assign that, you should see divs without height.
The most common solution is to set the following property in css:
html{
height: 100%;
margin: 0;
padding: 0;
}
You are saying to the html tag (html is the parent of all the html elements) "Take all the height in the HTML document"
I hope I helped you. Cheers
