Hacker News Reader

Yes.

-x-column-width: 700px;

Unless you're reading a dense novel, that shouldn't really be much of a problem. Line length is only an issue when returning to the beginning of the next line, and only very long paragraphs will be problematic in this case. Text online usually varies a lot more than a novel - lists, quotes, images, etc. - and the page in question is no exception: it's just a list of captioned images for the most part.

Hacker News, even with its tiny font, is still perfectly readable at a width of 1680 because few comments have paragraphs that are more than 2 or 3 lines long.

Well, a 27" monitor isn't actually 27" wide, but I get your point.

No, full width lines get difficult to read, but locking me into some tiny width of text on my monitor is quite annoying and, as a default behaviour, obnoxious.

Better would be a % width, perhaps with a font size that follows.

And the absolute worst is a site that uses CSS that prevents my page zoom from functioning properly.