It is a "well-known fact" on the internet, based on my searching, but I couldn't find a definitive source on the matter, even though I'd swear I've seen one. (Perhaps I too am simply remembering the "well-known fact".) I even found several people asking for the definitive source and coming up empty.

The best justification (no pun intended) for using "ragged text" I found was under the heading "Text Alignment" on http://community.infragistics.com/ux/articles/text-treatment... .

It is also my opinion that full justification may make the text block as a whole prettier but makes the text significantly harder to read. However, soft hypens and zero-width spaces still have a role to play on the web even so because justified and ragged-right text both have problems with very, very long words, and on the internet multi-hundred character words actually show up with some frequency, mostly as URLs.

I've heard the "line identification hypothesis" mentioned a few times in regards to preferring ragged-right, but I'm pretty sure that's just a guess. It seems readers do find fully-justified text harder to read on the web sometimes, but I'd be willing to bet that it has a lot more to do with column width, leading and word/letter spacing. (Without appropriate hyphenation, word spacing can vary greatly in justified text, especially when lines are short-ish and words are long-ish.) I've found that a good screen-purposed serif typeface set with extra leading (1.2 em seems to be about right) and a line length of 35-40 ems works very well as justified text for "publications". Decent automated hyphenation ought to make things a lot better.