IE7錯誤：float：右側寬度爲100％，而不是元素大小

我有一個div內的兩個按鈕。一個需要被浮動權，需要有浮動左

<div class="btnwrapper"> 
    <div class="btnright"><a href="#" class="button"><span>Continue</span></a></div> 
    <div class="btnleft"><a href="#" class="button optionleft"><span>Back</span></a></div> 
    <div style="clear:both;"></div> 
</div>

和相應的CSS

.calculator .btnwrapper { width:607px; } 
.calculator .btnleft { float:left; border:1px solid green; } 
.calculator .btnright { float:right; border:1px solid red; } 
a.button { background:url(../images/bg-button-left.gif) no-repeat; float:right; height:29px; width:auto; padding:0 0 0 8px; display:block; color:#FFF; text-decoration:none; font-weight:bold; font-size:13px; cursor:pointer;} 
a.button span { background:url(../images/bg-button-right.gif) top right; height:16px; padding:8px 8px 5px 0px; display:block; width:auto; cursor:pointer; }

這是我得到的IE7的結果。所有其他現代瀏覽器都能正確處理。 IE7 Bug