Đặt outline style dạng hai đường liền (double) bằng CSS

Dùng outline-style: double.

Ví dụ

<html>
    <head></head>
    <body>
        <p style="outline-width:7px; outline-style:double;">
            This text is having 7px double outline.
        </p>
    </body>
</html>