form在两个div之间会有间距 得加上style="margin:0px"

  <div style="background-color: Black">
        ssssss
    </div>
    <div style="background-color:Olive">
        <form id="form1" runat="server" style="margin:0px">
        <div style="background-color: Yellow">
            xxxxxxxxxxxxxxxxxxxx
        </div>
        </form>
    </div>