Css--margin

Margin is the outer margin of the element box.

value	Description
Auto	Browser calculates margins
Length	Specify the value of the margin in specific units, such as pixels, centimeters and so on. The default value is 0px
%	Margin that specifies a percentage based on the width of the parent element
Inherit	Specifies that the margin should be inherited from the parent element

Where auto and inherit are 2 deterministic values that represent the values that the browser calculates, which represents the value that inherits the margin from the parent element. And in the middle of the 2 units length and%, the specific value is uncertain, can be

http://www.smallni.com/negative-margin/

http://www.zhihu.com/question/20116963

Css--margin