HTML是網頁開發中最重要的一環，它可以實現各種實用的功能，如倒計時功能。下面將介紹如何利用HTML代碼實現倒計時功能。
<!DOCTYPE html>
<html>
<head>
<title>倒計時功能</title>
<meta charset="UTF-8">
</head>
<body>
<h1>倒計時</h1>
<p id="demo">還剩下</p>
<script>
// Set the date we're counting down to
var countDownDate = new Date("Jan 1, 2022 00:00:00").getTime();
// Update the count down every 1 second
var x = setInterval(function() {
// Get today's date and time
var now = new Date().getTime();
// Find the distance between now and the count down date
var distance = countDownDate - now;
// Time calculations for days, hours, minutes and seconds
var days = Math.floor(distance / (1000 * 60 * 60 * 24));
var hours = Math.floor((distance % (1000 * 60 * 60 * 24)) / (1000 * 60 * 60));
var minutes = Math.floor((distance % (1000 * 60 * 60)) / (1000 * 60));
var seconds = Math.floor((distance % (1000 * 60)) / 1000);
// Output the result in an element with id="demo"
document.getElementById("demo").innerHTML = days + "天 " + hours + "時 "
+ minutes + "分 " + seconds + "秒 ";
// If the count down is over, write some text
if (distance < 0) {
clearInterval(x);
document.getElementById("demo").innerHTML = "倒計時結束！";
}
}, 1000);
</script>
</body>
</html>以上代碼中，定義了一個countDownDate變量，用于存儲倒計時結束的日期。然后通過setInterval函數，每隔一秒鐘就更新一下距離這個時間還有多久，最終將結果輸出到id為"demo"的p標簽中。
通過上述代碼，可以輕松地實現HTML倒計時功能，讓網頁變得更加實用。