From the linked W3 Candidate Recommendation above:
The viewport-percentage lengths are relative to the size of the initial containing block. When the height or width of the initial containing block is changed, they are scaled accordingly.
These units are vh (viewport height), vw (viewport width), vmin (viewport minimum length) and vmax (viewport maximum length).
How can this be used to make a divider fill the height of the browser?
For this question, we can make use of vh: 1vh is equal to 1% of the viewport's height. That is to say, 100vh is equal to the height of the browser window, regardless of where the element is situated in the DOM tree:
The p tag here is set to 100% height, but because its containing div has 200 pixels height, 100% of 200 pixels becomes 200 pixels, not 100% of the body height. Using 100vh instead means that the p tag will be 100% height of the body regardless of the div height. Take a look at this accompanying JSFiddle to easily see the difference!