window.onload = function () {
        var lis = document.getElementsByTagName('li');
        lis[0].style.backgroundColor = 'gold';
        lis[1].style.backgroundColor = 'gold';
    }

document.getElementsByTagName();标签获取元素

循环

1.for循环

window.onload = function () {
        var olist = document.getElementById('list01');
        var ali = olist.getElementsByTagName('li');

         for(var i=0;i<8;i++){
            if(i%2 == 0){
              ali[i].style.backgroundColor = 'gold';
            }
        }
        i++;
    }

2.while循环

var i=0;
 while (i<ali.length){
      if(i%2 == 0){
          ali[i].style.backgroundColor = 'gold';
                 }
           }
           i++;
           };

3.do while循环

var i = 0;
        do {
            if (i % 2 == 0) {
                ali[i].style.backgroundColor = 'gold';
            }
            i++;
        } while (i < ali.length);
    }

while和do while的区别:
do while是大括号里无论如何都先执行一次，执行完再判断是否成立
while是先判断是否成立，然后在执行大括号的内容