Setting a percentage value to border-width

[wmm]

I'm trying to chop a triangle off the bottom of a \[code\]div\[/code\] by using the CSS \[code\]border\[/code\] "hack". (Creating a 0 width and height object and giving it a big ol' border and making one side of the border transparent and the other sides a solid color). My problem is that the \[code\]div\[/code\] in question has a percentage width. Consequently my border needs to have a percentage width as well (and possible height which is another potential problem as the \[code\]div\[/code\] doesn't have a specified height). But, the css \[code\]border-width\[/code\] property doesn't seem to support percentage values. Alternative solutions to the problem of "chopping" a triangle off the bottom of a responsive \[code\]div\[/code\] would also be okay. Before someone recommends using an image or appending one I can't because the whole thing has a pattern and it wouldn't match up. Also I don't want to use multiple background images due to compatibility.