How do I get the div horizontally centered?

The 100 degree div is centered, most of it is an answer, but sometimes this method is not omnipotent ... No nonsense, will I know the methods are listed well, update at any time.

1, set the width value, specify margin-left and margin-right for auto (or margin:0 auto; (The way most people know, don't give an example)

2, set width, using Margin-left for the width of the negative half and relative positioning to achieve horizontal center (width and Margin-left can set the percentage)

1 #demo{2 width:500px;3 Height:300px;//height can not be set, use content to prop up height4 margin:0 0 0-250px;//(or directly set Margin-left:-250px;)5 position:relative;relative positioning6 Background-color:Pink;//easy to see effect7  Left:50%;8 Top:50%;9}

<id= "Demo"></div>

3. To be Continued ...

How do I get the div horizontally centered?