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.

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.