If the font's space width is 0 (which can happen with a very small font) then it's OK for the tab width to also be 0. See attachment 8485 [details] (bug 9062). Since 0 is a valid value, the uninitialized value should be something else.
Created attachment 8541 [details] Remove the assert and change the uninitialized value to -1 No layout test since DumpRenderTree forces a minimum font size.
Comment on attachment 8541 [details] Remove the assert and change the uninitialized value to -1 r=me