Luckily, designing this way shows how well various browsers have implemented CSS by now. When sticking to the guidelines you should see fairly consistent results across most modern browsers. Due to the sheer number of user agents on the web these days — especially when you factor in mobile — pixel-perfect layouts may not be possible across every platform. That’s okay, but do test in as many as you can. Your design should work in at least IE9+ and the latest Chrome, Firefox, iOS and Android browsers (run by over 90% of the population).