Web designers must simply accept that they are working in a medium that is not amenable to (sub)pixel-perfect reproducibility. It is not—and never has been—safe to assume that future versions of a given browser will render identically to the current one. It is not safe to assume that the browsers you are fine-tuning for will remain popular in the future. It is not safe to assume that other users with the same browser version (or a browser claiming to be the same version) as you are using will get an identical rendering as you get on your system.

As a web designer, you must accept that the browser is fundamentally not yours to control. It is an agent acting on behalf of the user, not on behalf of you.