JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <link rel="stylesheet" href="./index.css">
    <title>Document</title>
</head>

<body>
    <div id="h1Block">
        <h1>TO-DO-LIST</h1>
    </div>
    <div id="inputBlock">
        <div class="input-area">
            <input type="text" id="input" placeholder="請輸入待辦事項...">
            <br>
            <button onclick="inputBtn()" class="inputBtn">Submit</button>
            <button onclick="exportBtn()" class="exportBtn">Export</button>
            <select onchange="changeInputColor()" name="" id="priorSelect">
                <option id="" value="normal">Normal</option>
                <option id="important" value="important">Important</option>
                <option id="urgent" value="urgent">Urgent</option>
            </select>
        </div>
    </div>
    <hr>
    <div id="outputBlock">
        <ul id="list">

        </ul>
    </div>
    <script src="./index.js"></script>
</body>

</html>

CSS

body {
    background-color: darkslategray;
    /* background-repeat: none; */
    background-size: cover;
}

#h1Block {
    color: whitesmoke;
    font-size: larger;
    font-family: Fantasy;
    font-weight: bolder;
    /* font-style: italic; */
    text-decoration: underline;
    text-align: center;
}

#inputBlock {
    margin-top: 10px;
    margin-bottom: 10px;
    text-align: center;
}

#input {
    width: 35%;
    height: 32px;
    font-size: 24px;
    border-radius: 10px;
}

.redFontClass {
    color: red;
}

.orangeFontClass {
    color: orange;
}

.blackFontClass {
    color: black;
}

.inputBtn {
    border: 0px;
    font-size: 24px;
    color: black;
    border: 3px solid darkred;
    border-radius: 8px;
    margin: 20px;
}

.inputBtn:hover {
    cursor: pointer;
    background-color: darkred;
    color: white;
}

.exportBtn {
    border: 0px;
    font-size: 24px;
    color: black;
    border: 3px solid blueviolet;
    border-radius: 8px;
    margin: 20px;
}

.exportBtn:hover {
    cursor: pointer;
    background-color: blueviolet;
    color: white;
}

#outputBlock {
    font-size: 24px;
    color: white;
    display: block;
    width: 36%;
    margin-left: 30%;
    margin-right: 10%;
    align-items: center;
}

#list {
    list-style-type: none;
}

#list li {
    padding: 5px;
    border-bottom: 1px dashed whitesmoke;
    display: flex;
}

#list span {
    width: 90%;
}

#list button {
    border-radius: 5px;
    border: 0 px;
    width: 20%;
    border: 3px solid black;
}

#list button:hover {
    cursor: pointer;
    background-color: rgb(36, 35, 35);
    color: white;
}

/* 新增事件急迫性分類功能 */
#priorSelect {
    font-size: 24px;
    margin: 20px;
    border: 3px solid black;
    border-radius: 5px;
}

.important {
    color: orange;
}

.urgent {
    color: red;
}

#important {
    background-color: orange;
}

#urgent {
    background-color: red;
}

JavaScript

function inputBtn() {
    const listText = document.getElementById("input").value;
    var list = document.getElementById("list");
    var elemSpan = document.createElement('span');
    var elemLi = document.createElement('li');
    var elemBtn = document.createElement('button')

    let prior = document.getElementById('priorSelect');
    let index = prior.selectedIndex;
    let priorText = prior.options[index].text;

    elemSpan.textContent = listText;
    elemBtn.textContent = "Del";
    list.append(elemLi);
    elemLi.append(elemSpan);
    elemLi.append(elemBtn);

    if (priorText == "Important") {
        elemSpan.className = 'important';
    }

    if (priorText == "Urgent") {
        elemSpan.className = 'urgent';
    }

    elemBtn.onclick = delBtn;
}

function delBtn() {
    event.target.parentElement.remove();
}

function exportBtn() {
    let listText = document.getElementById('list').children;
    let exportText = "待辦事項: ";
    let num = 1;

    for (var x of listText) {
        if (x.children[0].className == 'important') {
            exportText = exportText + '(' + num.toString() + ')*' + x.children[0].textContent + "* ";
        } else if (x.children[0].className == 'urgent') {
            exportText = exportText + '(' + num.toString() + ')**' + x.children[0].textContent + "** ";
        } else {
            exportText = exportText + '(' + num.toString() + ')' + x.children[0].textContent + " ";
        }
        num = num + 1;
    }
    alert(exportText);
}
function changeInputColor() {
    let elemInput = document.getElementById('input');
    elemInput.classList.remove('orangeFontClass');
    elemInput.classList.remove('redFontClass');
    elemInput.classList.remove('blackFontClass');
    switch (event.target.value) {
        case 'important':
            elemInput.className = elemInput.className + 'orangeFontClass';
            break;
        case 'urgent':
            elemInput.className = elemInput.className + 'redFontClass';
           ...