HTML/CSS : Create a side border that is thin at top and thick at bottom (trying to avoid using border images)

I want to create a side panel with a right border that is almost 0px at the top but gradually increases to 5px at the bottom.

I am trying to avoid border images as I want to use least of external resources such as images.

How can I do that. I tried to search a lot for this but couldnot find it. So if somebody can point me to a similar question , that is also helpful.

Below is the link: http://ift.tt/1FI2Fzi

If you notice, the left is a div with a beige color border on left panel at right(left to image) which is having variable length.