Padding and Margin issues, esp with responsive video embeds

I waited until 2.5.2 was out having read about issues with 2.5.0 and everything seems to be technically working, but whatever you have done has badly messed with the layout in ways I can’t really understand. Anyone using a child theme and/or custom CSS to accurately line up content in differently structured rows, beware! It has already taken me a couple of hours on a realtively small site, laboriously going through and fixing things, and I haven’t finished yet – left and right margins that were perfectly lined up down the page, now vary and will need row by row manual correction.

The worst is responsive video embeds (using a very standard method – absolute iframe inside zero-height relative container, sized using vertical padding), where I’ve had to manipulate things apparently randomly until they work — padding values that work fine on one page have had to be different on other pages where I KNOW I simply replicated layouts, so it’s all very weird. Anyway, just a heads-up, as this caught me completely by surprise and my client wasn’t exactly thrilled either.
🙁