Written on 2015-09-16
When reading a book, you read a page at a time, top to bottom. Accordingly the text can, and should, be a unified block, to balance the appearance of the two opposed pages. No shape is required beyond the frame of the page.
But on the screen there is only one column of text, and you read not by turning pages, but by scrolling. And every time you scroll, you suffer a brief period of disorientation: you have to find the line where you stopped reading. Even if you use page-up and page-down, you are still scrolling by pixels, not lines, and usually have to make small adjustments to keep the bottom or top line within the viewport.
The worst form of scrolling-induced disorientation is when you arrive near the bottom of a page without realizing it and try to scroll down: I, for one, tend to lose my place completely. In fact TBRSS provides for this particular problem by adding extra vertical whitespace to the end of every article so it is always possible to scroll down without bottoming out.
To deal with scrolling, the reader needs more cues than they would need when reading a book. Spacing between paragraphs helps, because it makes the individual paragraphs easier to recognize. Even when using indentation, some spacing helps: use it, Bringhurst be damned. But ragged right is essential, because the line endings form a shape that is easy to recognize when the whole page is scrolled down.
So: in any medium where the reader reads by scrolling – instead of within the frame of a page – prefer ragged right, because it helps the reader keep their place as they scroll.
(Whether reading by scrolling is a good idea is outside the scope of this post; it is what we have.)