Why You Should Never Center Align Paragraph Text
uxmovement.com
uxmovement.com
A minute of Googling gave me a more researched article with more comprehensive suggestions. See 3.5 on p. 390.
http://core.ecu.edu/engl/tpc/MennoMenno/ftp/williams%202000.... (Link fixed, thanks!)
And then their poll is an exact example of this. I swear, every UX movement article I've seen has had an example of what they're campaigning against elsewhere in the page.
I ask: why? Headings are a different section of the page, a different conceptual item than the paragraph they relate to. Centering them, and giving them a different left-edge helps differentiate them from merely large text.
They told you. It will look off-center. Draw the lines on the right side of the text that they drew on the left above that and you'll see it. If the box is 20 units wide, the center of the centered text will be put at 10 units by your layout algorithm, but that will actually be to the right of the "actual" center, because nearly all lines of text will actually be shorter than 20 units. The 20 is an upper bound, not an actual bound.
I would be intrigued to see if they say the same thing if one justifies the text, but my guess is that they would tell you not to justify your text on the web anyhow.
Try the same experiment with a wider (say, 6-800px) block of text, such as you see more frequently than the ~200px they demonstrate with, and try to tell how off-center it is. Or with a longer block of text, where the width is more easily visible because the ragged edge on the right will inevitably get close to the actual width of the container. Or with anything with surrounding images / colors that are different, where it'll clearly be centered.
Meanwhile, left-align everything and then compare with a center-aligned header, and decide which one reads more easily. This is "UX movement", ie "user experience", not Designer Daily; ease of reading and detecting different sections of the page are part of UX.
Also, apparently dyslexic people have a particularly hard time reading justified text, but I don't have any good sources for that.
If the reason for this is that screens are lower resolution than print, I would expect reading justified text on screens to become easier over the next few years, as 300dpi screens first become available and then common.
http://en.wikipedia.org/wiki/File:The_river_effect_in_justif...
But "never"? I'm sure gorgeous counterexamples are only a few hops away.
Reminds me of militant opinions on font choice for resumes.