1. 給父級定義height

.parent {
    background-color: red;
}
.float-left {
    width: 200px;
    height: 200px;
    float: left;
}

<div class="parent">
      <div class="float-left"></div>
</div>

2. 在結尾處新增空div，樣式設定為clear: both

.parent {
    background-color: red;
}
.float-left
 
 {
    width: 200px;
    height: 200px;
    float: left;
}

<div class="parent">
    <div class="float-left"></div>
    <div style="clear: both"></div>
</div>

3. 父級定義偽類

.parent {
    background-color: red;
}
.float-left {
    width: 200px;
    height: 200px;
    float
 
: left;
}
.clearfloat:after {
    display: block;
    clear: both;
    content: '';
    visibility: hidden;
    height: 0;
    zoom: 1;
}

<div class="parent clearfloat">
    <div class="float-left"></div>
</div>

4. 把父級變成BFC

生成BFC的條件

1. 根元素
2. float的值不為none
3. overflow的值不為visible
4. display的值為inline-block、table-cell、table-caption
5. position的值為absolute或fixed

.parent {
    background-color: red;
    float: left;
}

.parent {
    background-color: red;
    overflow: hidden;
}

.parent {
    background-color: red;
    display: table-cell;
}

.parent {
    background-color: red;
   	position: absolute;
}