DrDoc - 11:00 pm on May 7, 2013 (gmt 0)
When setting font sizes, 1em = 100%. So you can use whichever form is easiest for you to read and remember.
I, respectfully, disagree:
Rather than figuring out how to avoid individual bugs or how to "fix" a nasty bug, by following the advice of using 100% for body and using em elsewhere you spare yourself from head aches.
In a browser, margins of up to 10% can be nice. (Look at the present page.*) Drop down to tablet size and you're wasting real estate if you take that much horizontal space out of action; continue down to phone size and you'll want hardly any margin at all. It may be relevant that with smaller devices-- tablet or phone --there is no window, but the device itself has a physical "margin" outside the text area. This becomes part of the viewing experience.
Sound advice! However, many/most don't know how to do this. But, yes, the whole layout (not just margins) needs to be modified explicitly for tablets and smart phones.
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