HTML

<div class="ly">
  <div class="ly__item">xxx</div>
  <div class="ly__item">xxx</div>
  <div class="ly__item">xxx</div>
  <div class="ly__item">xxx</div>
</div>

多个元素在一行

CSS

.ly{
    display: flex;
}

多个元素水平居中对齐

.ly{
    display: flex;
    justify-content: center;
}

多个元素水平两端对齐

.ly{
    display: flex;
    justify-content: space-between;
}

多行多个元素水平两端对齐

.ly{
    display: flex;
    flex-wrap: wrap;
    justify-content: space-between;
}

多个元素在一行，某个元素占据剩余部分

.ly{
    display: flex;
}
//给占据剩余部分的元素加该类名
.ly__item--fill{
    flex-grow: 1;
}

多个元素水平居右对齐

.ly{
    display: flex;
    justify-content: flex-end;
}

多个元素垂直居中

.ly{
    display: flex;
    align-items: center;
}

多个元素水平垂直居中对齐

.ly{
    display: flex;
    justify-content: center;
    align-items: center;
}