Another one of those things where more blame is put on the developer instead of the UA. Zooming or not zooming, breaking layouts or not, these are relative measures to judge if px or em is better. You can base your text on em and you can still have the same issues: too large or too small text on the page, breaking layouts on zoom. These are false arguments. In the end, using px is a choice a developer should be able to make. The UAs need to get better at it. End of story.
It's not enough to be able to set the base font and size in the UAs settings, like changing it from the default 16px to 18px. The UA needs to have simple options for text settings overrides. Like Dolphin does, when it's letting you set the text size to: tiny, small, normal, large, huge. That's it. A very good place to start.
There are two sides of accessibility, in your developer's yard, like in don't break the accessibility for a bigger number of people just to satisfy a smaller part, and leave both parties helpless.
If there is one thing a developer needs to do is to be able to intelligently use media queries. That's it.