lucy24 - 7:51 am on Jul 21, 2012 (gmt 0)
Zooming overrides everything including pixels. Think of images: you've set their height and width explicitly. But if the user does a full-page zoom the image gets bigger too.
But please, please don't set a font size in pixels. Especially not for body text. Different users have different preferences-- and often different needs, depending on their age and eyesight. That's why browsers let you set a default font size. This is the size you get in your CSS when you say "font-size: medium" or (in isolation) "font-size: 100%". The pixel size also varies according to individual font; sans-serif fonts almost always "read" bigger than serif fonts of the same point size. So my default font is 15pt Palatino but my default sans-serif font is 13pt.
There's a nice illustration somewhere in w3's CSS documentation, but it may take some searching. It's under "font-size-adjust" which was dumped from CSS 2.1 but mercifully restored in CSS 3.
Also, please don't use <br> routinely. There are rare situations where it is appropriate,* but do not repeat do not use it to delimit paragraphs. That's what <p>...</p> is for.
* I recently landed on three different <br> situations in rapid succession-- but they were all very unusual and specialized, not something you'd do every day. Unless you are in the habit of making e-books based on originals that are in prose with numbered lines used by critical apparatus and a massive Glossarial Index so you have to keep the line breaks, but they're obviously not paragraphs.
The second one involved multi-column lists of numbers where each number was supposed to be a new line but some were too short and tried to squeeze in with the preceding number.
I, uhm, forget the third. But I'm sure it was equally obscure.