请老师检查谢谢

<!DOCTYPE html>
<html>

<head lang="en">
    <meta charset="UTF-8">
    <title>输入文本</title>
</head>

<body>
    <p>字数限制在30字内，<span>您还可以输入<b id="count">30</b> 字</span></p>
    <textarea cols="50" rows="7" id="text"></textarea>
    <br><button type="submit" id="btnSubmit"
        style="width: 120px;height:30px;background-color: green;font-size: 18px;color:white">提交</button>
    <script type="text/javascript">
        //补充代码
        var text = document.getElementById("text");
        var count = document.getElementById("count");
        var btnSubmit = document.getElementById("btnSubmit");
        text.onkeyup = function () {
            var len = text.value.length;
            var total = 30;
            var allow = total - len;
            if (allow < 0) {
                count.innerHTML = allow;
                btnSubmit.innerHTML = "禁止提交";
                btnSubmit.style.cursor = "not-allowed";
                btnSubmit.style.backgroundColor = "red";
                btnSubmit.setAttribute("disabled", "disabled");
            } else {
                count.innerHTML = allow;
                btnSubmit.innerHTML = "提交";
                btnSubmit.style.cursor = "default";
                btnSubmit.style.backgroundColor = "green";
                btnSubmit.removeAttribute("disabled")
            }
        }
    </script>
</body>

</html>

慕UI4313976 2020-05-08

源自：JS DOM事件 4-5 编程练习