Defines the space outside the element, on the top side.
默认属性
margin-top: 0;
Removes any margin at the top.
margin-top: 30px;
You can use pixel values.
margin-top: 2em;
You can use (r)em values.
The value is relative to the font size:
<html> the root element's font size
margin-top: 10%;
You can use percentage values.
The percentage is based on the width of the container.