How to distinguish between large categories when the javascript drop-down box is not selected_Form special effects

<title>runcode</title> <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> <meta name="Author" content="Sheneyan"> <select> <optgroup label="a"> <option>a1</option> <option>a2</option> <option>a3</option> </optgroup> <optgroup label="b"> <option>b1</option> <option>b2</option> <option>b3</option> </optgroup> </select>
[Ctrl A select all Note: If you need to introduce external Js, you need to refresh to execute ]