Web Design in a Nutshell, 3rd Edition

Border thickness

You can use the nonstandard border attribute in the frameset element to specify the thickness of the frame borders in pixels. The default thickness varies by browser. Although border is not part of the Frameset DTD, it is fairly well supported by current browsers.

Tip

Turning the frameborder off removes only the 3D border, but it leaves a gap between the frames. To remove this gap and give the page a smooth, seamless appearance, use the border attribute with a setting of 0 pixels: <frameset frameborder="0" border="0">