The discussion over whether pixels (px) or ems (em) are best as a way to measure content when designing web pages is often ended with the argument that all modern browsers can scale content defined in pixels, which is not untrue.
What falls short from this argument is that it puts the burden on the user to make the conscious decision to change the font size. The mental process could go like this:
I'm having trouble reading this. I'm squinting. The text is too small. I'll press... what is it? oh, ctrl,+. One, two, three... no, one less so ctrl,-. There.
The triviality of the example is balanced by the importance of avoiding disruption when it comes to user interaction. An action that we require from a user (in this case, attention to what we write) is impoverished by the division of said attention between the task that we want performed and adjusting the viewport.
The whole point of affordance and usability is precisely to take the burden of thinking from the user, so it becomes an enjoyable activity. Would I like my readers to enjoy my blog? yes. Would I like my buyers to enjoy purchasing on my website? yes. Would you?
An additional element is missing from pixels: There is research regarding the ideal width for a line of text (which I defer to you to investigate and debate). When using pixels, the rhythm and proportion of your page is much more difficult to preserve, since now it is a matter of font size and line width. If instead the layout is all dependent on font size, a person designing and coding a website can have a lot more control over the aesthetics without additional cognitive burden.
Author tweaks site css...
*ahem*. Got it?