I think it is a bug

Original file:
[frameset cols="175,*,220"]
  [frame name="menu" src="homemenu.html" scrolling= "no" /]
  [frame name="content" src="intro.html" /]
[frame name="whatsnew" src="whatsnew.html" /]
[/frameset]

After httptrack


[frameset cols="175,*,220"]
  [frame name="menu" src=http://intro.html/homemenu.html scrolling= "no" /]
  [frame name="content" src=http://intro.html/intro.html /]
[frame name="whatsnew" src=http://intro.html/whatsnew.html /]
[/frameset]

This happened for HTTrack version 3.43-9+libhtsjava.so.2 and