使用无单位数值给line-height赋值
无单位数值乘以本元素的字体大小才是最终的line-height值，这个可以避免，由于line-height继承带来的问题。
line-height给各种类型元素带来的影响
On block level elements, the line-height property specifies the minimum height of line boxes within the element.
On non-replaced inline elements, line-height specifies the height that is used to calculate line box height.
On replaced inline elements such as buttons or other input elements, line-height has no effect.
