It turns out that, as with so many of the things we “know” are right, the idea that serif typefaces are more readable than non-serif typefaces simply isn’t supported by the evidence.
Between Instapaper, The Magazine, and this site, I’ve done a lot of experimenting with different typefaces and styles to try to optimize for readability.1 And I’ve found surprisingly few universal rules — my choices always depend on screen size, pixel density, text size, and background color.
In my anecdotal experimentation, serifs are more readable in some contexts, and sans-serifs are more readable in others. I like Lyon Text (serif) on Retina iPad, Elena (serif) on iPad Mini, and Ideal Sans on iPhone. I don’t like dark backgrounds with light text anywhere except on iPhone, and only with sans-serifs.
And Instapaper’s customers choose a wide variety of typefaces and settings. Elena (serif) is the most popular font among people who ever change it, but Proxima Nova (sans) is often in second place. Tisa (serif) is very popular on iPhone, where I don’t care for serifs as much. And Android renders fonts so “differently2” that almost no good iOS or Mac font choices yield good results.
A typeface’s readability is about far more than just one cosmetic attribute.