[css] Can I position an element fixed relative to parent?

I know this is an older post, but I think a good example of what Jiew Meng was trying to do can already be found on this site. Check out the side menu located here: https://stackoverflow.com/faq#questions. Looking at it without getting into it too deep, I can tell javascript attaches a fixed position once the scrolling hits below the anchor tag and removes the fixed positioning if the scrolling goes above that same anchor tag. Hopefully, that will get someone started in the right direction.