<div style="width:200px; height:40px">

            <div class="yiji" style="background-color:red;" onclick="Show('g')">公告</div>

            <div class="yiji" style="background-color:#F96;" onclick="Show('x')">新闻</div>

        </div>

        <div class="erji" id="g" style="background-color:red">

        	<span>wwwwwwwwwwwwwwwww</span>

            <span>wwwwwwwwwwwwwwwww</span>

            <span>wwwwwwwwwwwwwwwww</span>

            <span>wwwwwwwwwwwwwwwww</span>

        </div>

        <div class="erji" id="x" style="background-color:#F96; display:none;">

        	<span>ddddddddddd</span>

            <span>ddddd</span>

            <span>sss</span>

        </div>

*{

				margin:0px auto;

				padding:0px;

			}

			.yiji{

				width:100px;

				height:40px;

				float:left;

				text-align:center;

				line-height:40px;

				vertical-align:middle;

				}

			.erji{

				width:200px;

				height:200px;

				}

function Show(a)

		{

			var a = document.getElementById(a);

			var e = document.getElementsByClassName("erji");

			//回复原样

			for(var i=0;i<e.length;i++)

			{

				 e[i].style.display = "none";

			}

			a.style.display = "block";

		}

<div id="kuang">

        	<div id="jindu" style="width:0px;"></div>

        </div>

#kuang{

				width:500px;

				height:20px;

				border:1px solid #999;

				}

			#jindu{

				height:20px;

				background-color:#3F0;

				float:left;

				}

Zou();

		window.setInterval("Zou()",20);

		function Zou()

		{

			var j = document.getElementById("jindu");

			//获取元素的长

			var str = j.style.width;

			//获取宽度里的数字

			var cd = str.substr(0,str.length-2);

			if(cd<500)

			{

				cd = parseInt(cd)+2;

				j.style.width = cd+"px";

			}

		}