Frame Borders
By default, framed documents display with a 3D border between each frame. These borders visually divide the sections and also serve as a handle for resizing. The HTML 4.01 specification allows for borders to be controlled only at the frame level (in the frame element).
Tip
Most browsers also support the nonstandard method of setting borders and border thicknesses for the whole page in the frameset element. Bear in mind that this nonstandard use of border attributes will cause a document to be invalid because it does not conform to any DTD.

Figure 14-6. Effects of setting frame margins
Browsers vary in their support of border attributes. It is best to do plenty of testing (including in older browsers) to be sure you can live with the different results.