so far I used em for all my margins. When I change the size of the font (by the zoom buttons on the ebook reader), I want that things like margins or text intend also change that way.
Where is the big difference between em and % ?
Talking about default font size: I just made a test with my Bebook. I took the same html file and used 3 different css files.
In the first try I did not set the body font size, I used the default of the reader. In the 2nd try, I set the base font to 1em. And with the 3rd try, I set it to 20 px.
The results: First of all: like I expected, setting no base fontsize gave the same result like setting it to 1em. The text was awfull small. I wear glasses (but the last time I had my eyes checked was less then 6months ago). I could read the text (but it wasn't easy), but I know someone like my 65 year old father would have needed a looking glass (or the zoom buttons on the Bebook)
Then I looked at some full lines with no intend (just to add: the text was justified, but contains no long words on this lines). The first line had like 87 characters, the next that I tried was 82.
When I zoomed in 1 level, it looked much better and I had around 64 characters on a line.
So I looked at the 3rd test, the one with base font set to 20px. The font size itself was much better, about 2-2.5 milimeters high. I also looked at some lines: The first one had 70 characters, the next one had 66.
I don't know, but for me, it seems as if I should use something like 20-22 (or maybe even 24) px as font size for my device.
However, if I use a different device ... oh man, everything is so complicated ...