CSS居中之 { left:50%； top:50%； transform:translate(-50%,-50%)； }

CSS居中之 { left:50%; top:50%; transform:translate(-50%,-50%); }

left:50%; top:50%; transform:translate(-50%,-50%);

left:50%; top:50%; transform:translate(-50%,-50%);

也可以写成: left:50%; top:50%; translate: -50% -50%;

left:50%; top:50%; translate: -50% -50%;

• 用 translate 的必须是块级元素, 或者absolute,fixed 才会生效
  比如 translate 在 relative+inline 时不会起作用

• left 和 top 在 position:relative,absolute,fixed 才会生效

所以position不能是static ; display不能是inline,

• relative+inline 时, left和top起作用, translate不起作用

• relative+block 时, relative使得left和top起效 , block使得translate起效

方法1: 不用设置父元素, 被居中元素 position:relative, 必须是块元素[ block | inline-block | flex | inline-flex | grid | inline-grid | table | inline-table | …等块级元素 ] , 不能是 inline

.被居中元素{position:relative; display:block; left:50%; top:50%; transform:translate(-50%,-50%);}

.被居中元素{position:relative; display:inline-block; left:50%; top:50%; transform:translate(-50%,-50%);}

.被居中元素{position:relative; display:grid; left:50%; top:50%; transform:translate(-50%,-50%);}

.被居中元素{position:relative; display:inline-grid; left:50%; top:50%; transform:translate(-50%,-50%);}

.被居中元素{position:relative; display:flex; left:50%; top:50%; transform:translate(-50%,-50%);}

.被居中元素{position:relative; display:inline-flex; left:50%; top:50%; transform:translate(-50%,-50%);}

.被居中元素{position:relative; display:table; left:50%; top:50%; transform:translate(-50%,-50%);}

.被居中元素{position:relative; display:inline-table; left:50%; top:50%; transform:translate(-50%,-50%);}

方法2: 父元素设置position:< relative | absolute | fixed> , 被居中元素position:absolute或fixed, 不能是relative , 可以不用设置display

.父元素{position:relative;}
.被居中元素{position:absolute; left:50%; top:50%; transform:translate(-50%,-50%);}

.父元素{position:absolute;}
.被居中元素{position:absolute; left:50%; top:50%; transform:translate(-50%,-50%);}

.父元素{position:fixed;}
.被居中元素{position:absolute; left:50%; top:50%; transform:translate(-50%,-50%);}

.父元素{position:relative;}
.被居中元素{position:fixed; left:50%; top:50%; transform:translate(-50%,-50%);}

.父元素{position:absolute;}
.被居中元素{position:fixed; left:50%; top:50%; transform:translate(-50%,-50%);}

.父元素{position:fixed;}
.被居中元素{position:fixed; left:50%; top:50%; transform:translate(-50%,-50%);}

transform:translate(-50%,-50%) 可以写成 translate -50% -50%

.被居中元素{position:relative; display:block; left:50%; top:50%; translate -50% -50%;}

.父元素{position:relative;}
.被居中元素{position:absolute; left:50%; top:50%; translate -50% -50%;}

例子

<div id="B" style="width: 800px; display: grid; grid: auto / 10fr 10fr">
  <div
    class="居中容器"
    style="border: 1px solid black; width: 100%; height: 200px"
  >
    <span
      class="居中元素"
      style="
        position: relative;
        left: 50%;
        top: 50%;
        translate: -50% -50%;
        width: 200px;
        height: 100px;
        font-size: 66px;
        border: 1px solid red;
        text-align: center;
      "
      >不居中
    span>
  div>
  <label>父元素::<br />子元素:: position:relative;label>
  <div
    class="居中容器"
    style="
      position: relative;
      border: 1px solid black;
      width: 100%;
      height: 200px;
    "
  >
    <span
      class="居中元素"
      style="
        position: absolute;
        left: 50%;
        top: 50%;
        translate: -50% -50%;
        width: 200px;
        height: 100px;
        font-size: 66px;
        border: 1px solid red;
        text-align: center;
      "
      >居中
    span>
  div>
  <label>父元素::position:relative;<br />子元素:: position:absolute;label>
  <div
    class="居中容器"
    style="border: 1px solid black; width: 100%; height: 200px"
  >
    <span
      class="居中元素"
      style="
        position: relative;
        display: block;
        left: 50%;
        top: 50%;
        translate: -50% -50%;
        width: 200px;
        height: 100px;
        font-size: 66px;
        border: 1px solid red;
        text-align: center;
      "
      >居中
    span>
  div>
  <label>父元素::<br />子元素:: position:relative; display:blocklabel>
  <div
    class="居中容器"
    style="border: 1px solid black; width: 100%; height: 200px"
  >
    <span
      class="居中元素"
      style="
        position: relative;
        display: inline-block;
        left: 50%;
        top: 50%;
        translate: -50% -50%;
        width: 200px;
        height: 100px;
        font-size: 66px;
        border: 1px solid red;
        text-align: center;
      "
      >居中
    span>
  div>
  <label>父元素::<br />子元素:: position:relative; display:inline-blocklabel>
  <div
    class="居中容器"
    style="border: 1px solid black; width: 100%; height: 200px"
  >
    <span
      class="居中元素"
      style="
        position: relative;
        display: flex;
        left: 50%;
        top: 50%;
        translate: -50% -50%;
        width: 200px;
        height: 100px;
        font-size: 66px;
        border: 1px solid red;
        text-align: center;
      "
      >居中
    span>
  div>
  <label>父元素::<br />子元素:: position:relative; display:flexlabel>
  <div
    class="居中容器"
    style="border: 1px solid black; width: 100%; height: 200px"
  >
    <span
      class="居中元素"
      style="
        position: relative;
        display: inline-flex;
        left: 50%;
        top: 50%;
        translate: -50% -50%;
        width: 200px;
        height: 100px;
        font-size: 66px;
        border: 1px solid red;
        text-align: center;
      "
      >居中
    span>
  div>
  <label>父元素::<br />子元素:: position:relative; display:inline-flexlabel>
  <div
    class="居中容器"
    style="border: 1px solid black; width: 100%; height: 200px"
  >
    <span
      class="居中元素"
      style="
        position: relative;
        display: grid;
        left: 50%;
        top: 50%;
        translate: -50% -50%;
        width: 200px;
        height: 100px;
        font-size: 66px;
        border: 1px solid red;
        text-align: center;
      "
      >居中
    span>
  div>
  <label>父元素::<br />子元素:: position:relative; display:gridlabel>
  <div
    class="居中容器"
    style="border: 1px solid black; width: 100%; height: 200px"
  >
    <span
      class="居中元素"
      style="
        position: relative;
        display: inline-grid;
        left: 50%;
        top: 50%;
        translate: -50% -50%;
        width: 200px;
        height: 100px;
        font-size: 66px;
        border: 1px solid red;
        text-align: center;
      "
      >居中
    span>
  div>
  <label>父元素::<br />子元素:: position:relative; display:inline-gridlabel>
  <div
    class="居中容器"
    style="border: 1px solid black; width: 100%; height: 200px"
  >
    <span
      class="居中元素"
      style="
        position: relative;
        display: table;
        left: 50%;
        top: 50%;
        translate: -50% -50%;
        width: 200px;
        height: 100px;
        font-size: 66px;
        border: 1px solid red;
        text-align: center;
      "
      >居中
    span>
  div>
  <label>父元素::<br />子元素:: position:relative; display:tablelabel>
  <div
    class="居中容器"
    style="border: 1px solid black; width: 100%; height: 200px"
  >
    <span
      class="居中元素"
      style="
        position: relative;
        display: inline-table;
        left: 50%;
        top: 50%;
        translate: -50% -50%;
        width: 200px;
        height: 100px;
        font-size: 66px;
        border: 1px solid red;
        text-align: center;
      "
      >居中
    span>
  div>
  <label>父元素::<br />子元素:: position:relative; display:inline-tablelabel>
div>