结合 writing-mode 属性，为元素设置 border-start-start-radius 边框圆角

实例代码

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <title>垂直书写模式下 border-start-start-radius 演示</title>
    <style>
        body {
            font-family: 'Microsoft YaHei', sans-serif;
            padding: 20px;
            display: flex;
            flex-direction: column;
            align-items: center;
        }
        
        .box {
            width: 200px;
            height: 120px;
            margin: 30px 0;
            padding: 15px;
            background-color: #f0f8ff;
            display: flex;
            align-items: center;
            justify-content: center;
            text-align: center;
            border: 2px solid black;
            writing-mode: vertical-rl;
            border-start-start-radius: 25px; /* 修正为 start-start 以匹配标题 */
        }
        
        .code {
            background-color: #f5f5f5;
            padding: 10px;
            border-radius: 5px;
            font-family: monospace;
            margin: 15px 0;
            width: 80%;
            max-width: 300px;
        }
        
        .note {
            color: #666;
            font-size: 14px;
            text-align: center;
            margin-top: 10px;
            max-width: 300px;
        }
        
        .highlight {
            color: #d63384;
            font-weight: bold;
        }
    </style>
</head>
<body>
    <h2>垂直书写模式下的 border-start-start-radius</h2>
    
    <div class="code">
.example {
    border: 2px solid black;
    writing-mode: vertical-rl;
    border-start-start-radius: 25px; /* 与CSS中的属性保持一致 */
}
    </div>
    
    <div class="box">
        垂直从右到左书写模式<br>
        border-start-start-radius: 25px
    </div>
    
    <div class="note">
        在 <span class="highlight">vertical-rl</span> (从右到左的垂直书写模式)下：
        <br><br>
        <span class="highlight">border-start-start-radius</span> 作用于：
        <br>
        • <strong>逻辑上</strong>：文本流开始端的起始角
        <br>
        • <strong>物理上</strong>：右上角
        <br><br>
        (在 vertical-rl 模式下，起始端是右侧)
    </div>
</body>
</html>