content-box和border-box

refer to

http://www.w3school.com.cn/cssref/pr_box-sizing.asp

refer to

https://blog.csdn.net/qq_32862143/article/details/79393987

• content-box 
  说明：在内容宽度和高度之外绘制内边距和边框

• border-box 
  说明：在已设定的宽度和高度之内绘制设定元素的边框及内边距