翻看html5 classList API：

myDiv.classList.add('myCssClass'); // 添加 class  
myDiv.classList.remove('myCssClass'); // 移除 class  
myDiv.classList.toggle('myCssClass'); // 切換 class  

直接上代碼吧：

<ul id="runner_list" class="runner_list">
    <li id="runner1" class="runner">1</li>
    <li id="runner2" class="runner">2</li>
    <li id="runner3" class="runner">3</li>
    <li id="runner4" class="runner">4</li>
</ul>

window.onload=function(){
        var runnerList=document.getElementById("runner_list"),
            btnGo=document.getElementById("go"),
            btnReady=document.getElementById("ready"),
            runnerArr=Array.prototype.slice.call(runnerList.querySelectorAll("li"));

        btnGo.onclick=function(){
            runnerArr.forEach(function(i){
                i.classList.add('go');
            });
        };
    }