Today when I tried something with background option, I tried to put value none and then validated it and didn't receive error. How come?
In official specifications as I understod property background can't have value none or can it?
The 'background' property is a shorthand property for setting the individual background properties (i.e., 'background-color', 'background-image', 'background-repeat', 'background-attachment' and 'background-position') at the same place in the style sheet.
Given a valid declaration, the 'background' property first sets all the individual background properties to their initial values, then assigns explicit values given in the declaration.
background-image: none; is a valid declaration and so is seen as setting just the image to none when used in the shorthand background
means a value for 'background-image' has been given and the other individual properties are set to their initial value.