请问一下这两道题代码怎么写

img

img

第一个

<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1" />
    <title> 页面名称 </title>
<style type="text/css">
#tbox {
    display: flex;
    width:300px;
    margin:0;
    padding: 0;
    list-style:none;
    flex-flow: column nowrap;
}
#cbox {
    display: flex;
    width: 100%;
    height: 200px;
    margin:0;
    padding: 0;
    list-style:none;
    flex-flow: row nowrap;
}
li {
    border: 1px solid #999;
}
#cbox li {
    width: 100px;
}

</style>
</head>
<body>

<ul id="tbox">
    <li>header</li>
    <li>
        <ul id="cbox">
            <li>aaa</li>
            <li>bbb</li>
            <li>ccc</li>
        </ul>
    </li>
    <li>footer</li>
</ul>

</body>
</html>

第二个

用@media screen

/*页面宽度>500的样式*/
#xxx {}

@media screen and (max-width: 500px) {

    /*页面宽度<=500的样式*/
    #xxx {}
    
}

媒体查询