The problem that I see is that by making the decision that something needs to be pixel perfect the designer is often "volunteering" the engineer for an hour or more of fighting with CSS. In my experience the engineer is then expected to spend that time in addition to their normal workload.
The only ways I can see it working are either:
- Designers are obligated to follow the DLS.
- Any divergence from the DLS is pointed and therefore takes away from the number of points the engineer commits to for other tasks during a sprint.