之前和之後的div不顯示

我想製作一個具有截止角的div。我想用a：before和after元素做背景圖像。位於div的左側和右側。但他們不顯示...之前和之後的div不顯示

這裏是我的html：

<div class="page"> 
     <div class="canvas" id="canvas"> 
      <div class="TimeLineContainer"> 
       <div class="TimelineContent"></div> 
      </div> 

     </div> 
    </div>

這是使用CSS IAM：

html, body { 
      width: 100%; 
      height: 100%; 
      margin: 0; 
      padding: 0; 
     } 

     body { 
      font-size: 100%; 
      font-family: "Calibri", Arial, sans-serif; 
      color: black; 
      text-align: center; 
      overflow: hidden; 
     } 

     .page { 
      display: block; 
      text-align: left; 
      width: 100%; 
      height: 100%; 
      position: relative; 
      overflow: hidden; 
     } 

      .page .canvas { 
       display: block; 
       text-align: left; 
       width: 100%; 
       height: 100%; 
       position: absolute; 
       overflow: hidden; 
       top: 0; 
       left: 0; 
      } 

      .page .canvas .TimeLineContainer { 
       display:block; 
       width:100%; 
       height:150px; 
       text-align: center; 
       overflow: hidden; 
      } 

       .page .canvas .TimelineContent { 
        display: block; 
        width:90%; 
        margin: 0 auto; 
        height:150px; 
        position: relative; 
        background-image: url(../img/middleTop.png); 
        background-repeat: repeat-x; 
        overflow: auto; 
       } 

       .page .canvas .TimeLineContainer .TimeLineContent:before { 
        position:absolute; 
        content:""; 
        top:0px; 
        left:-28px; 
        background-image: url (../img/cornerBL.png); 
        background-repeat: no-repeat;  
        display: block; 
        width: 28px; 
        height: 150px; 
       } 

       .page .canvas .TimeLineContainer .TimeLineContent:after { 
        position:absolute; 
        content:""; 
        top:0px; 
        right:-28px; 
        background-image: url (../img/cornerBR.png); 
        background-repeat: no-repeat; 
        display: block; 
        width: 28px; 
        height: 150px; 
       }

這些圖像IAM使用：

cornerBL.png