I've never met a designer who wasn't completely fine with my suggestions for more pragmatic solutions. Like just styling a default scrollbar instead of implementing my own scrollbar to make it exactly like the design. Using a default drop-down menu instead of rolling my own just so I can round the corners of the selects.
The designers I've worked with are fine with these things. We have more important things to work on than small style details. We can go back and change these things later if anyone actually cares, but generally nobody ever does.
I've never met a designer who cares how it gets done but I have hard time believing they were OK with the corners not being rounded as per the design. They may agree on shipping without the rounded corner, as long as the ticket to round that corner is registered.
I suppose though that we have just had very different life experiences, as that is what the HN guidelines would require of us.