By default, browsers use the standard box model. 
默认情况下，box-sizing: content-box 标准的box模式。

If you want to turn on the alternative model for an element you do so by setting box-sizing: border-box on it. 

box-sizing: content-box/border-box
表示 width包含内容和内边距，或者只包含内容

html {
  box-sizing: border-box;
}
*, *::before, *::after {
  box-sizing: inherit;
}