> Ragged-right was the norm for wider media such as printed books

What? I'm pretty sure that if I pick any book in my shelf, it's going to be justified.

> Most web media are narrow-column format, so tend to be fully justified.

What #2? 99% of web media is ragged-right, the biggest reason being that it's the default, and that browsers have terrible line-wrapping and historically had no support for hyphenation. And justified text gets worse the shorter the lines are, because there are fewer options on where to insert newlines, leading to large spaces between words. Also, good justification requires fine-grained word spacing control, which doesn't work well with traditional low-resolution displays.

My MSc thesis advisor recently told that apparently thesis documents should be submitted with ragged-right lines these days because it makes them easier to read for dyslexics; while it makes sense, it must be a quite new guideline.

ragged right became the web norm because browsers lacked good hyphenation, making justified text look gappy. In print it works fine, but online ragged right tends to be easier to read.