padding absolute

<style>
    .box{
        position: relative;
        width: 100px;
        height: 100px;
        border: 1px solid green;
    }
    .big-box{
        width: 400px;
        height: 200px;
        border: 1px solid red;
        padding: 20px;
        box-sizing: border-box;
    }
    .box2{
        /*float: left;*/
        position: absolute;
        left: 0;
    }
    .box3{
        float: left;
        background-color: #8d7afb;
    }
</style>
<div class="box big-box">
    <div class="box box2"></div>
<!--    <div class="box box3"></div>-->
</div>

absolute left不会计算padding
当就一个物体向左移动时，float、绝对定位都可以，但多个物体对齐时，用float结合父元素的padding，或者新建子元素div，配合它的margin、padding

绝对定位实现float效果

<style>
    .box{
        position: relative;
        width: 100px;
        height: 100px;
        border: 1px solid green;
    }
    .big-box{
        width: 400px;
        height: 200px;
        border: 1px solid red;
        padding: 20px;
        box-sizing: border-box;
    }
    .box2{
        /*float: left;*/
        position: absolute;
        /*left: 0;*/
        right: 20px;
        background-color: red;
    }
    .box3{
        float: left;
        background-color: #8d7afb;
    }
</style>
<div class="box big-box">
    <div class="box box2"></div>
    <div class="box box3"></div>
</div>

<style>
    .box{
        position: relative;
        width: 100px;
        height: 100px;
        border: 1px solid green;
    }
    .big-box{
        width: 400px;
        height: 200px;
        border: 1px solid red;
        padding: 20px;
        box-sizing: border-box;
    }
    .box2{
        /*float: left;*/
        position: absolute;
        /*left: 0;*/
        right: 20px;
        background-color: red;
    }
    .box3{
        float: left;
        background-color: #8d7afb;
    }
    .box4{
        float: right;
        border: 5px solid green;
    }
</style>
<div class="box big-box">
    <div class="box box2"></div>
    <div class="box box3"></div>
    <div class="box box4"></div>
</div>