What is the difference between background-size: cover; and background-size: 100%;?

here's a fiddle: http://jsfiddle.net/RS5kX/19/

background-size:100%; = background-size:100% auto; = the width is set to be 100% large and the height of the background image follows respecting the image aspect ratio.

background-size:cover; means the background image will always fit the whole div , you won't be left with any empty spots in your div

background-size:100% 100%

won't leave any empty space too, but of course this will detroy the original image aspect ratio

Pretty sure background-size: cover; means the image will fill the element while maintaining its aspect ratio, while background-size: 100%; will just make the image fill 100% width of the element.

cover = Scale the background image to be as large as possible so that the background area is completely covered by the background image. Some parts of the background image may not be in view within the background positioning area

Basically it zooms in until the inner most edges are touching the side, which means that some of the image may be cut off unlike 100% where all of the image will be visible.

If it did not do the zoom in, you would end up with two sides that reach the edge but on the other axis you would have blank horizontal (or vertical) looking 'bars' on either side of the image in one of those directions.

Your Question: Why would they looks the same ?
Answer: If the image / container are square

See http://www.w3schools.com/cssref/playit.asp?filename=playcss_background-size&preval=cover for example