fixed定位问题

position：fixed定位是相对视口宽度，即使在里面嵌套一层div，其定位也是display：fixed，在对其的宽度定位为20%时，也是相对于整个视口,而不是相对于父级元素

待研究：怎样让dispaly:fixed的定位是相对于父级，而不是整个视口

 .ScreeningConditions {
     
        width: 100%;
        height: 100%;
        position: fixed;
        top: 0;
        left: 0;
        z-index: 3;
    }
    .ScreeningConditionsLeft {
        width: 20%;
        height: 100%;
        background-color: black;
        filter: alpha(opacity=50);
        -moz-opacity: 0.5;
        -khtml-opacity: 0.5;
        opacity: 0.5;
    }
    .ScreeningConditionsRight {
        width:80%;
        height:100%;
        background-color:white;
        position:relative;
    }
  
  
    .ScreeningConditionsFoot {
        width:100%;
        text-align:center;
        position:fixed;
        bottom:0;
        height:40px;
        line-height:40px;
       
    }
    .ScreeningConditionsFootReset {
        width:30%;
        background-color:#d4d4d4;
        float:left;
    }
    .ScreeningConditionsFootOK {
        width: 50%;
        float:left;
        background-color: red;
    }

<div class="ScreeningConditions">
    <div class="ScreeningConditionsLeft fl"></div>
    <div class="ScreeningConditionsRight fl">
        <footer class="ScreeningConditionsFoot">
            <div class="ScreeningConditionsFootReset">重置</div> <div class="ScreeningConditionsFootOK">确认</div>
        </footer>
    </div>
</div>

当ScreeningConditionsFootOK中width为70%时：