行内元素的水平居中(两种)

1. text-align: center

    <div class="parent">
        <span class="child">content</span>
    </div>

.parent{
            background-color: red;
            text-align: center;
     }

2. width: fit-content; margin: auto;

    <div class="parent">
        <span class="child">content</span>
    </div>

.parent{
            background-color: red;
            width: fit-content;  //父元素的宽度自适应，由子元素撑开
            margin: auto;
        }

单行文本的垂直居中(一种)

1. line-height

    <div class="parent">
        <span class="child">content</span>
    </div>

.parent{
            background-color: red;
            height: 200px;
            line-height: 200px;
        }

块级元素的水平居中

1. margin: 0 auto;

    <div class="parent">
        <div class="child"></div>
    </div>

        .parent{
            background-color: red;
            
        }

        .child{
            background-color: blue;
            width: 100px;
            height: 100px;
            margin: 0 auto;
        }

水平垂直居中

1. 定位+ margin-top + margin-left

需要提前知道子元素的宽高，取子元素的宽高的一半来移动

    <div class="parent">
            <div class="child"></div>
    </div>

        .parent{
            background-color: red;
            width: 400px;
            height: 400px;
            position: relative;
        }

        .child{
            background-color: blue;
            width: 100px;
            height: 100px;
            position: absolute;
            left: 50%;
            top: 50%;
            margin-top: -50px; //子元素宽的一半
            margin-left: -50px; //子元素高的一半
        }

2.定位 + calc()函数

    <div class="parent">
            <div class="child"></div>
    </div>

        .parent{
            background-color: red;
            width: 400px;
            height: 400px;
            position: relative;
        }

        .child{
            background-color: blue;
            width: 100px;
            height: 100px;
            position: absolute;
            left: 50%;
            top: 50%;
            left: calc(50% - 50px); 
            top: calc(50% - 50px);
        }

3. 定位+transfrom

不需要知道子元素的宽高，也可实现

    <div class="parent">
            <div class="child"></div>
    </div>

        .parent{
            background-color: red;
            width: 400px;
            height: 400px;
            position: relative;
        }

        .child{
            background-color: blue;
            width: 100px;
            height: 100px;
            position: absolute;
            left: 50%;
            top: 50%;
            transform: translate(-50%,-50%)
        }

4. 定位 + margin

    <div class="parent">
        <div class="child"></div>
    </div>

        .parent{
            background-color: red;
            width: 400px;
            height: 400px;
            position: relative;
        }

        .child{
            background-color: blue;
            width: 100px;
            height: 100px;
            position: absolute;
            left: 0;
            top: 0;
            right: 0;
            bottom: 0;
            margin: auto;
        }

5. padding

    <div class="parent">
        <div class="child"></div>
    </div>

        .parent{
            background-color: red;
            padding: 20px;
        }

        .child{
            background-color: blue;            
            height: 100px;  
        }

6. flex

    <div class="parent">
        <div class="child"></div>
    </div>

        .parent{
            background-color: red;
            width: 400px;
            height: 400px;
            display: flex;
            justify-content: center;
            align-items: center;
        }

        .child{
            background-color: blue;      
            width: 100px;      
            height: 100px;  
        }

7. grid

    <div class="parent">
        <div class="child"></div>
    </div>

        .parent{
            background-color: red;
            width: 400px;
            height: 400px;

            display: grid;
        }

        .child{
            background-color: blue;      
            width: 100px;      
            height: 100px;  

            align-self: center;
            justify-self: center;
        }