The picture in the "measure" section has a red "x" over 99 characters per line and suggests 45 to 75 characters per line. But the article's paragraphs have about 95 characters per line.
Or not. Such experimental evidence as we have on this point -- which isn't much for a sensible scientific/evidence-based debate -- doesn't necessarily support a claim that wider line widths will objectively reduce reading speed or comprehension/retention, at least not until you get a lot wider. There may be an argument that readers subjectively prefer mid-width lines, though.
In any case, it may be that the optimal comfort width is dictated by more physically-based factors, such as the angle we can comfortably scan moving only our eyes and not also turning our heads, and that letter counts are just a proxy for this based on typical text sizes and reading distances (meaning that it may or may not be a good proxy since the recent shift to much larger web fonts).
It's almost impossible to say anything definitive about line width on web pages because the experience depends so much on viewport aspect ratio, device rotation, and surrounding content.
The x-words-per-line standard derives from print, which has different traditions. Calling them rules is probably unwise. (See e.g. small column widths in newspaper and magazine layouts, vs paperback novel layouts, vs etc.)
Blog pages seem to have standardised on a single central column between a half and two thirds of the viewport width. Text sizing covers a huge range from ~12pt up to 16pt or even 18pt.
Modern product pages seem to have standardised on a selection of standard Bootstrap layouts, but with a lot of holdouts - e.g. Amazon - who do their own thing.
I'm not aware of any hard evidence about comprehension/retention metrics for these layouts. But they're very popular, and visitors don't seem to hate them.
There's actually more evidence for unexpected conversion rate increases from apparently insignificant changes to button positioning, icon colours, and such.
Much as I love beautiful typography, I think it's only a deal-breaker when it goes horribly wrong. For many sites, other design elements are likely more important.
It's almost impossible to say anything definitive about line width on web pages because the experience depends so much on viewport aspect ratio, device rotation, and surrounding content.
I agree. We know that margin width seems to make a difference in other contexts, so it seems likely that on small mobile devices where there is very little margin the other characteristics for optimum readability might need to be adjusted to take that into account. Clearly there is an inherent limit on the physical width of lines we can show on such devices anyway.
Blog pages seem to have standardised on a single central column between a half and two thirds of the viewport width. Text sizing covers a huge range from ~12pt up to 16pt or even 18pt.
It would be nice to see some real evidence about the effectiveness of single-column blog layouts, but first we'd have to choose some plausible alternatives to compare them with. There don't seem to be many "standardised" ones today, as other heavily text-based sites that use things like multi-column layouts tend to be in fields like news or e-commerce, where they aren't trying to present a single piece.
It would also be interesting to see a proper study about the effectiveness or otherwise of very large fonts and of thin/light fonts, as widely used on trendy blogs today. I suspect they would not be a resounding success, but I have nothing beyond anecdotal evidence to back that up.
Much as I love beautiful typography, I think it's only a deal-breaker when it goes horribly wrong.
Sadly, that is true of much design work. When you get it right, your site works well and no-one notices. When you get it wrong, it can be disastrous. Such is life. :-)
Do as I say, not as I do.