z-index: https://developer.mozilla.org/zh-CN/docs/Web/CSS/z-index
层叠上下文:https://developer.mozilla.org/zh-CN/docs/Web/Guide/CSS/Understanding_z_index/The_stacking_context
https://www.zhangxinxu.com/wordpress/2016/01/understand-css-stacking-context-order-z-index/
硬件加速:https://www.html5rocks.com/en/tutorials/speed/layers/
合成:https://fed.taobao.org/blog/taofed/do71ct/performance-composite/