Recently I have noticed a disturbing thing: websites with excessive line lengths (i.e. the amount of characters on a single line of text). Some websites sport line length between 90 and 190! That’s a lot of characters.
Anything from 45 to 75 characters is widely regarded as a satisfactory length of line for a single-column page set in a serifed text face in a text size. The 66-character line (counting both letters and spaces) is widely regarded as ideal.
From The Elements of Typographic Style Applied to the Web
I haste to say, the vast majority of websites are doing just fine. But some … not good at all. Strangely, every single one of these belongs to people with design credentials. And yet, it appears they skipped their typography class.
Culprit is most likely responsive design. A single column layout is easier to code fiddling with other layout furniture. I know, I am guilty of that too. But that does not justify ignoring a simple typography rule.
So there. Watch those line lengths. Please.