    <!DOCTYPE html>
    <html>
    <head>
      <meta charset="UTF-8">
      <title>스타일 변경하기</title>
      <link rel="stylesheet" href="style_js.css">
    </head>
    <body>
      <div id="box" style="width: 300px; height: 300px; margin: auto"></div>
      <script>
        const letters = "0123456789ABCDEF";
        let color = "#";
        let count = 1;
        let intervalID = setInterval(changeColor, 250);

        function changeColor() {
          if (++count > 50)
            clearInterval(intervalID);
          for (let k = 0; k < 6; k++)
            color = color + letters[Math.floor(Math.random() * 16)];
          document.getElementById("box").style.backgroundColor = color;
          document.getElementById("box").style.borderRadius = `${count}%`;
          color = "#";                              // 16진수 문자열 초기화
        }
      </script>
    </body>
    </html>