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The default value of the background-color is transparent. It means no color. It means 'no color' It means "no color" It means the height of the image falls back to its default value 'auto' The background-size property is the measurement of the width of the element that is going to be filled by the image. The height of a background image can be set back to the original aspect ratio to keep the image's aspect ratio. If you do not care about aspect ratio then you can follow this method to always contain your whole image as a background.