function validateForm() {
    var x = document.forms["myForm"]["fname"].value;
    if (x == null || x == "") {
        alert("需要輸入名字。");
        return false;
    }
}

<form name="myForm" action="demo_form.php" onsubmit="return validateForm()" method="post">
名字: <input type="text" name="fname">
<input type="submit" value="提交">
</form>

<form action="demo_form.php" method="post">
  <input type="text" name="fname" required="required">
  <input type="submit" value="提交">
</form>

function validateForm()
{
  var x=document.forms["myForm"]["fname"].value;
  if (x==null || x=="")
  {
    alert("姓必須填寫");
    return false;
  }
}

<form name="myForm" action="demo-form.php" onsubmit="return validateForm()" method="post">
姓: <input type="text" name="fname">
<input type="submit" value="提交">
</form>

function validateForm(){
  var x=document.forms["myForm"]["email"].value;
  var atpos=x.indexOf("@");
  var dotpos=x.lastIndexOf(".");
  if (atpos<1 || dotpos=x.length){
    alert("不是一個(gè)有效的 e-mail 地址");
    return false;
  }
}</atpos+2>

<form name="myForm" action="demo-form.php" onsubmit="return validateForm();" method="post">
    Email: <input type="text" name="email">
    <input type="submit" value="提交">
</form>

<input id="id1" type="number" min="100" max="300" required>
<button onclick="myFunction()">驗(yàn)證</button>
 
<p id="demo"></p>
 
<script>
function myFunction() {
    var inpObj = document.getElementById("id1");
    if (inpObj.checkValidity() == false) {
        document.getElementById("demo").innerHTML = inpObj.validationMessage;
    }
}
</script>

<input id="id1" type="number" max="100">
<button onclick="myFunction()">驗(yàn)證</button>
 
<p id="demo"></p>
 
<script>
function myFunction() {
    var txt = "";
    if (document.getElementById("id1").validity.rangeOverflow) {
       txt = "輸入的值太大了";
    }
    document.getElementById("demo").innerHTML = txt;
}
</script>

setCustomValidity('') 
setCustomValidity(null) 
setCustomValidity(undefined)

var inpObj = document.getElementById("id1");
inpObj.setCustomValidity(''); // 取消自定義提示的方式
if (inpObj.checkValidity() == false) {
    if(inpObj.value==""){
        inpObj.setCustomValidity("不能為空！");
    }else if(inpObj.value<100 || inpObj.value>300){
        inpObj.setCustomValidity("請(qǐng)重新輸入數(shù)值（100~300之間）!");
    }
    document.getElementById("demo").innerHTML = inpObj.validationMessage;
} else {
    document.getElementById("demo").innerHTML = "輸入正確";
}

(function(){
       if(window.addEventListener) window.addEventListener("load",init,false);
       else if(window.attachEvent) window.attachEvent("onload",init);
        
      function init(){
       for(var i=0;i)
       {
           var f=document.forms[i];//第i個(gè)form
           var needValidation = false;// Assume ,for now,that this form does not need any validation
            
           for(var j=0;j)
           {
               e = f.elements[j];
                
               if(e.type!="text") continue;
               var pattern = e.getAttribute("pattern");
               var required = e.getAttribute("required")!=null;
               //See if it has attribute that require validation
               if(required&&!pattern){
               pattern = "<a  rel="external nofollow"   target="_blank">\\s</a>"; 
               e.setAttribute("pattern",pattern);
               }
 
               if(e.pattern){
               e.onchange = validateOnChange;
                
               needsValidation = true;
               }
          }
       if(needValidation) f.onsubmit = validateOnSubmit;
        }
        }
      function validateOnChange()
        {   var textfield = this;
            var pattern = textfield.getAttribute("pattern");
            var value = textfield/*this*/.value;
             
            if(value.search(pattern)==-1) textfield.classname = "invalid";
            else textfield.classname = "valid";
        }
      function validateOnSubmit()
        {
            var invalid = false;
            for(var i=0;i<this.elements.length;i++)
               {
                  var e = this.elements[i];
                  if(e.type =="text" && e.onchange == validateOnChange){
                    e.onchange();
                    if(e.classname == "invalid") invalid = true;
                   }
                }
            if(invalid){
             alert("The form is incompletely or incorrectly filled out.\n "+"Please correct the highlighted field and try again");
             return false;
            }
        }
})();</f.elements.length;j++</document.forms.length;i++

<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Untitled Document</title>
<style >
    input.invalid { background-color:#fa0;}
</style>
</head>
 
<body>
<form action="test.asp">
Name:<input type="text" name="name" required  /><br />
email:<input type="text" name="email" pattern="^\s*\w+@\w+\.\w+\s*$" /><br />
zipcode:<input type="text" name="zip" pattern = "^\s*\d{5}\s*$" />

unvalidated:<input type="text" onchange='alert("test")' /><br />
<input type="submit" />
</form>