frameborder = 1|0 [CN]
    This attribute provides the user agent with information about the frame border. Possible values:
 

• 1: This value tells the user agent to draw a separator between this frame and every adjoining frame. This is the default value.
• 0: This value tells the user agent not to draw a separator between this frame and every adjoining frame. Note that separators may be drawn next to this frame nonetheless if specified by other frames.
  

12.9. How do I remove the border around frames?
 
Removing the border around frames involves both not drawing the frame borders and eliminating the space between the frames. The two major frames-capable browsers use different proprietary attributes to achieve this.
 
Netscape recognizes the BORDER attribute on FRAMESET. It can be set to 0, in which case the border will not be shown, and the spacing will be set to zero.
 
Microsoft Internet Explorer recognizes the FRAMEBORDER and FRAMESPACING attributes on FRAMESET, but in some versions also on FRAME for individual frames. Both attributes must be set to 0.
 
So, the most widely supported way to display borderless frames is <FRAMESET ... BORDER=0 FRAMEBORDER=0 FRAMESPACING=0>.
 
Note that these attributes are proprietary and not part of the HTML 4 specifications. Also, removing the border around a frame makes it impossible to resize it, as this border is also used in most GUIs to change the size of the window.