https://segmentfault.com/a/1190000019207842 关于移动端适配，你必须要知道的
device pixel ratio简称dpr-devicePixelRatio:1
视觉视口(visual viewport)-innerWidth:1280
outerWidth:1280
布局视口(layout viewport)-document.documentElement.clientWidth:1280
screen.width:1280
width: 100px
width: 300px
width: 100dp
width: 100pt
width: 50%
width: 50vw