css3 flex弹性布局登录页面

/* 将body标签转换为弹性容器 */
display: flex;
/* 主轴方向设置为垂直交叉轴 */
flex-direction: column;
/* 内容不换行 */
flex-wrap: nowrap;
/* 弹性元素水平方向居中显示 */
justify-content: center;
/* 设置弹性容器中弹性元素居中显示 */
align-items:center;

<div class="container">
  <h3>管理员登录</h3>
  <form>
    <div><label for="">用户名:</label><input type="text" name="" id=""></div>
    <div><label for="">密&emsp;码:</label><input type="password" name="" id=""></div>
    <div><button>登录</button></div>
  </form>
</div>

* {
  margin: 0;
  padding: 0;
}

body,
html {
  width: 100%;
  height: 100%;
}

body {
  color: #444;
  font-size: 14px;
  font-family: '微软雅黑';
  font-weight: lighter;
  background-image: linear-gradient(to top, lightcyan, white, lightcyan);
  /* 弹性布局 */
  /* 将body标签转换为弹性容器 */
  display: flex;
  /* 主轴方向设置为垂直交叉轴 */
  flex-direction: column;
  /* 内容不换行 */
  flex-wrap: nowrap;
  /* 弹性元素水平方向居中显示 */
  justify-content: center;
  /* 设置弹性容器中弹性元素居中显示 */
  align-items: center;
  /* 将body标签垂直划分一百份 */
  /* width: 100vh; */
}

h3 {
  /* 不加粗 */
  font-weight: normal;
  text-align: center;
  margin-bottom: 15px;
  font-size: 20px;
}

.container {
  width: 300px;
  padding: 20px;
  box-sizing: border-box;
  position: relative;
  box-shadow: 0 0 8px gray;
  border-radius: 8%;
}

.container form {
  display: flex;
  flex-flow: column nowrap;
  background-image: linear-gradient(to right bottom, lightblue, white);
  border-radius: 8px;
  padding: 15px;
  transition: 0.5s ease-in-out;
}

.container form:hover {
  box-shadow: 0 0 5px gray;
}

.container>form>div {
  display: flex;
  flex-flow: row nowrap;
  margin: 10px 0;
}

.container input {
  flex: 1;
  margin-left: 10px;
  border: 1px solid gray;
  outline: none;
  padding-left: 3px;
  height: 23px;
  line-height: 23px;
}

.container input:hover {
  outline: 0.5px solid gray;
}

.container button {
  flex: 1;
  height: 30px;
  line-height: 28px;
  background-color: skyblue;
  color: #fff;
  text-align: center;
  border: none;
  outline: none;
  cursor: pointer;
  letter-spacing: 15px;
  border-radius: 8%;
}

.container button:hover {
  background-color: steelblue;
}