This might be obvious to someone less ignorant in the mystic ways of CSS, but I thought it was worth sharing anyway.
I was struggling to work out a suitable font-size for a mobile web page. Whether I tried using size names like “x-large”, or values with em, pt, px or %, the same basic problem remained:
Font-sizes were either too small on smartphones with high-resolution screens, or too large on other smartphones.
If I increased the font-size to look better on a high-res screen, it was too big for other phones. If I decreased the font-size for those other phones, it looked tiny on a high-res screen.