I've noticed some strange behavior with both Chrome and Safari on my Mac:
The page is a simple fixed div with some text in it, I added a second div that does a simple CSS translation over 5 seconds so you can easily see the difference. My web app is using CSS transitions to show and hide portions, and while it was doing this large portions of the screen seemed to shift.
I've placed example code and a .mov file out on a server so hopefully you will see the same issue:
http://physicaltable.com/index.html and http://physicaltable.com/CSS_JIGGLE_TEST_2.mov
The strange bolding doesn't occur in Chrome on Windows 7, nor does it happen on the iPad 2 (v5.1.1)
Has anyone else seen this type of issue?
I realize this isn't much of an answer, but I've found that it's mostly because of the rendering of the elements. If the element needs to use hardware or any other type of graphics rendering, it basically takes an image of the text, adjusts it, then rerenders the text (if it can).
The "taking a picture" is where the boldness is lost, since the browser/display/something is adding the flair that makes the text look good. Of course it doesn't look that good, but that's just me.
I've noticed with different colors other than all white/black, it can behave differently. I'm hoping things will get smoothed out as browsers and rendering advances.