I'm calling the following a "text group":
1. A singular comment
2. For a text submission such as Ask HN, the post contents
A general readability recommendation is to limit your line lengths to be around 45-80 (e.g. [1]).
For websites that just have a singular main body of text (e.g. blog posts, articles), this is easy to do with css rules.
The text on HN comments page is of a bit different nature. Each child comment cascades, there's an indent. So to apply the typography rule of "not too many characters in a line, one would have to apply the css rules per text group*. Effectively a child comment is still getting wrapped with the same column width, not lower.