SOURCE

function setFontStyle() {
    const fontStyle = document.getElementById('fontStyle').value;
    document.execCommand(fontStyle, false, null);
}

function saveContent() {
    const editor = document.getElementById('editor');
    const content = editor.innerHTML;
    console.log(content); // 输出富文本内容是HTML字符串
}

<div id="editor" contenteditable="true" style="border: 1px solid #ccc; min-height: 200px;"></div>

<select id="fontStyle">
    <option value="normal">Normal</option>
    <option value="bold">Bold</option>
    <option value="italic">Italic</option>
</select>
<button onclick="setFontStyle()">Set Style</button>