水平居中

1、子元素为行内元素：父text-align: center;

.parent {
    background-color: violet;
    text-align: center;
}
.child {
    background-color: yellow;
}

2、子元素为块级元素定宽：子margin: 0 auto

.parent {
    background-color: violet;
}
.child {
    width: 100px;
    background-color: yellow;
    margin: 0 auto;
}

3、子元素为块级元素：子position：absolute + trabsform

.child {
    background-color: yellow;
    position: absolute;
    left: 50%;
    transform: translateX(-50%);
}

4、子元素为块级元素定宽：子absolute + width + margin-left

.child {
    width: 100px;
    background-color: yellow;
    position: absolute;
    left: 50%;
    margin-left: -50px;
}

垂直居中

1、子元素为单行文本

.parent {
    background-color: violet;
    line-height: 100px;
}
.child {
    background-color: yellow;
}

2、display：table + vertical-align: middle

vertical-align: middle只在父层是td或th才生效

.parent {
    background-color: violet;
    height: 100px;
    display: table;
}
.child {
    background-color: yellow;
    display: table-cell;
    vertical-align: middle;
}

水平垂直居中

1、flex

.parent {
    background-color: violet;
    height: 100px;
    display: flex;
    justify-content: center;
    align-items: center;
}
.child {
    background-color: yellow;
}

2、子不定高

.parent {
    background-color: violet;
    height: 100px;
    position: relative;
}
.child {
    background-color: yellow;
    position: absolute;
    left: 50%;
    top: 50%;
    transform: translate(-50%, -50%);
}

3、子定高

.parent {
  background-color: violet;
  height: 100px;
  position: relative;
}
.child {
  background-color: yellow;
  position: absolute;
  left: 50%;
  top: 50%;
  width: 100px;
  height: 60px;
  margin-left: -100px;
  margin-top: -30px;
}

4、position: absolute; + margin: auto;

.parent {
    background-color: violet;
}
.child {
    background-color: yellow;
    width: 200px;
    height: 200px;
    position: absolute;
    left: 0;
    top: 0;
    right: 0;
    bottom: 0;
    margin: auto;
}

5、table-cell

.parent {
    background-color: violet;
    height: 100px;
    display: table-cell;
    text-align: center;
    vertical-align: middle;
}
.child {
    background-color: yellow;
}