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                section 下面是多列展示区域
p 样式里规定是3列, 但内容总数为偶数4, 则列数为2
  div 1
  div 2
  div 3
  div 4
p 下面添加一个内容, 则列数变3列, 因为内容总数为5, 是奇数 

button#add 点我添加5

section 但是看下面, 偶数一样可以三列, 又是怎么回事? 因为有一个子元素设置了
  code margin: 1px 0;

p 只要设置了上下边距中的一个, 就可以达到这样的效果, 设置左右边距是没有效果的

p 设置border也能达到同样的效果, 但padding则不行

  div.margin 1
  div 2
  div 3
  div 4
section 下面看看跨列对顺序的影响

p 这里设置的列数为2 数字3是跨列的
p 在Chrome55下, 排列顺序由从上到下, 变成从左到右了, Firefox则照旧. 这个顺序是依赖具体浏览器的实现

  div 1
  div 2
  div.cross-col 3
  div 4
  div 5


  -webkit-column-count 3
  -moz-column-count 3
  column-count 3
  // column-width 30vw
  // column-gap 1px
    // 加border也会变3列
    // border 1px dotted

  //有了这句话, 内容为偶数也就能变成3列了
  margin 1px 0
  -webkit-column-count 2
  -moz-column-count 2
  column-count 2
  -webkit-column-span all
  -moz-column-span all
  column-span all

  margin-bottom 10px
    content ''
    display block
    border-top 1px solid
    margin 10px 0


                window.onload = function(){
  document.getElementById('add').onclick = function() {
   var ele = document.createElement('div')
    ele.textContent = 5
    this.onclick = null