1、a标签的四个伪类:link,visited,hover,active,并且最好按照上面顺序来写,防止覆盖 2、清除浮动 3、数组展开 const flat = function(arr, depth) { return arr.reduce((res, cur) => { if(depth && cur instanceof Array) { return [...res, ...flat(cur, depth - 1)] } el...