一、如果元素的position属性值为absolute,那么此时分为两种情况:
1. 父级(包括直接父级和间接父级)元素中没有定义position属性(即默认属性static),那么该元素会参照页面,以页面为基准进行定位,top, left等属性会参照页面。
如果该元素的width和height属性是百分数的话,那么这两个属性也是参照页面的宽度和高度。
<!doctype html><html><head><meta charset="utf-8"><style>*{margin: 0; padding: 0;}#div1 { width: 200px; height: 200px; background: #ccc;}#div2 { width: 50%; height: 50%; background: #aaa; top: 100px; left: 100px; position: absolute;}</style></head><body><div id="div1"> <div id="div2"></div></div></body></html>
以上代码中,div2是绝对定位,而其父级元素没有设置position属性,那么该元素就会以页面为参照进行定位。
从图中可以看到,div2是相对于页面定位的,而且其宽度和高度是页面宽高的一半。
从控制台可以看到,页面的宽度和高度正好是div2元素的2倍。
2. 父级元素定义了position属性(relative或absolute),则会参照父级元素。
<!doctype html><html><head><meta charset="utf-8"><style>*{margin: 0; padding: 0;}#div1 { width: 200px; height: 200px; background: #ccc; position: relative; top: 100px; left: 100px;}#div2 { width: 50%; height: 50%; background: #aaa; top: 100px; left: 100px; position: absolute;}</style></head><body><div id="div1"> div1 <div id="div2">div2</div></div></body></html>
从图中可以看到,div2是相对于div1定位的,宽度和高度也是相对于div1
二、如果元素的position属性值为relative,那么该元素相对于其正常位置(即position:static)定位。
默认情况下宽度会占满父容器,高度根据本身内容决定。当是父元素的第一个子元素时,位置会在父元素的左上角。
那么设置其位置和大小就会以自己为参照,height的情况特殊,当设置height时,会以充满父容器的状态为参照。
<!doctype html><html><head><meta charset="utf-8"><style>*{margin: 0; padding: 0;}#div1 { width: 100px; height: 100px; background: #ccc;}#div2 { width: 50%; height: 50%; background: #aaa; top: 100px; left: 100px; position: relative;}</style></head><body><div id="div1"> <div id="div2"></div></div></body></html>

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