基本概念
顶线（top） 中线（middle） 基线（baseline） 底线（bottom）
行距（line-height）———— 两行文字任意一条线的距离
font-size ———— 顶线到底线
line-height = font-size + 半行距*2 = font-size + 行距
CSS3 object-fit/object-position - CR
Method of specifying how an object (image or video) should fit inside its box. object-fit options include “contain” (fit according to aspect ratio), “fill” (stretches object to fill) and “cover” (overflows box but maintains ratio), where object-position allows the object to be repositioned like background-image does.
不支持IE11
