I do this on my personal site and it seems to Just Work. I specify a max-width for my content, specify font sizes in em values, and it looks fine on every browser I've ever tried, even a shitty Chinese cellphone (not smartphone).
Why is that better?
If a user sets their default font size to 26px, then a max-width of 600px may be too small... if a user sets their font size to 10px then 600px may be too wide. A value of 35em would scale the container based on the user's selected font size.
Containers should be relative to their content.
Makes sense. Thanks!
max-width would probably be better for mobile/small screens.