It's a fair point, but it's hardly unique to Jupyter. In fact, while 99% of websites suffer from this problem, I think it's unfair to highlight it specifically

It's a fair point, but it's hardly unique to Jupyter. In fact, while 99% of websites suffer from this problem, I think it's unfair to highlight it specifically wrt Jupyter. Heck, even the site we're on right now does a poor combination of uncomfortably-long lines AND unused left and right margins.