<p>输入数字并点击验证按钮：</p>

<input type="number" id="nu" max="300" min="100" required>

<button type="button" onclick="myFunction()">点我</button>

<p>如果输入数字100-300以外，会提示错误信息。</p>

<p id="demo"></p>

function myFunction() {

  var x = document.getElementById("nu");

  x.setCustomValidity("");//使用前先取消自定义，否则下次点击checkValidity总返false

  if (x.checkValidity() == false) {

    x.setCustomValidity("错误");

    document.getElementById("demo").innerHTML = x.validationMessage;

  }

  else {

    x.setCustomValidity("正确");

    document.getElementById("demo").innerHTML = x.validationMessage;

  }

}