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CSS CSS Basics (2014) Basic Layout Backgrounds: Size and Position

Matthew Keys
Matthew Keys
1,922 Points

Explanation of the slash after the image position

Can someone please explain to me why you need to put a slash after the image position (center / cover)?

1 Answer

Hi Matthew,

This is a syntax requirement to remove ambiguity that would occur without it.

background-position and background-size can both accept some of the same types of values. For example, percentages and lengths.

Imagine the following background shorthand was used:

background: 30% 80%;

This can be interpreted multiple ways if we didn't have that forward slash syntax requirement. Are both of those values for the background-position? Are both for background-size? Is one for background-position and the other for background-size?

To remove this uncertainty, it was decided that background-position would be first, then a forward slash, and then the background-size.

This syntax also means that it's not possible to specify the background-size without also first specifying the background-position using the shorthand.

Going back to the earlier example, because the browser does not see a forward slash followed by values for background-size, it knows that the 30% 80% is for background-position. There is no other way to interpret it.