I'm not a fan of this project either (see my other comment), but I totally agree with them on this one. Justified text is really hard to get right, even with TeX, InDesign and the like, and simply placing "text-align: justify;" in your CSS will worsen the quality and therefore the readability: Gaps will occur between all words in every other line, which doesn't only look ugly, but also destroys the flow of reading.
Left justification is always preferable over bad full justification; I don't know many (read: any) typographers who would disagree.
The first of Tschichold's Penguin Composition Rules (always worth reading - http://s3.amazonaws.com/tsch/compositionrules.html) says: "All text composition should be as closely word-spaced as possible." By relying on automatic full justification, you violate that rule in cruel ways. "Wide spaces should be strictly avoided."
You'll be hard-pressed to find any wikipedia article that looks bad with it, unless you zoom in to something like 6 words per line on average.
Edit to your edit: I don't particularly care for your appeal to authority when experience has borne out for me that wildly varying line lenghts hinder reading considerably more than occasional gaps. Besides, your authority doesn't even mention justification or alignment, as such i question your binding his statement to that matter.
Your claims would hold more weight if you actually demonstrated them. Also it would be nice if you actually marked your copious edits into your post as edits.
Left justification is always preferable over bad full justification; I don't know many (read: any) typographers who would disagree.
The first of Tschichold's Penguin Composition Rules (always worth reading - http://s3.amazonaws.com/tsch/compositionrules.html) says: "All text composition should be as closely word-spaced as possible." By relying on automatic full justification, you violate that rule in cruel ways. "Wide spaces should be strictly avoided."