uniappはメソッド関数のスタイルを変更します

<template>
  <view>
    <form>
      <input type="text" placeholder="用户名" />
      <input type="password" placeholder="密码" />
      <button>登录</button>
    </form>
  </view>
</template>

<style scoped>
  form {
    width: 80%;
    margin: 10vw;
    padding: 5vw;
    background-color: #fff;
  }
  input {
    margin-bottom: 3vw;
    width: 100%;
    height: 10vw;
    border: 0;
    border-radius: 3vw;
    background-color: #f2f2f2;
  }
  button {
    width: 100%;
    height: 12vw;
    border: 0;
    border-radius: 3vw;
    color: #fff;
    background-color: #007aff;
    font-size: 4.5vw;
  }
</style>

<template>
  <view>
    <form>
      <input ref="username" type="text" placeholder="用户名" />
      <input ref="password" type="password" placeholder="密码" />
      <button @click="login">登录</button>
    </form>
  </view>
</template>

<script>
export default {
  methods: {
    login() {
      if (!this.$refs.username.value.trim()) {
        this.$refs.username.placeholder = '未填写'
        this.$refs.username.style.borderBottomColor = 'red'
      }

      if (!this.$refs.password.value.trim()) {
        this.$refs.password.placeholder = '未填写'
        this.$refs.password.style.borderBottomColor = 'red'
      }
    },
  },
}
</script>