If a frameset contains three or more frames, use a "longdesc" attribute to help describe the purpose of the frames.

Example

Wrong

<frameset cols="25%,50%,25%"> <frame src="frame_1.htm" /> <frame src="frame_2.htm" /> <frame src="frame_3.htm" /></frameset>

Right

<frameset cols="25%,50%,25%" longdesc="description.html"> <frame src="frame_1.htm" /> <frame src="frame_2.htm" /> <frame src="frame_3.htm" /></frameset>