When a finalized design concept is being applied to a new site, it is of course important to make sure that crucial design details are not lost in the process. To a large degree, this can simply be facilitated using a methodical approach to the application.
I have found, however, that often when I think a site is “perfect” there are actually a lot of elements that don’t quite line up with what the designer had in mind.
One method I’ve used to counter this is to add the original design source an an overlayed layer to the final site, one that can be toggled on and off. This allows me to verify that things like element placement, margins and padding, and even line height and font size are faithfully being reproduced.