[RussianCow]

Wouldn't this leave a bunch of white space on one side if the screen width is not an exact multiple of that size? That doesn't seem like a good solution to me.

[iSnow]

The browser could distribute the white space between columns to avoid this.

[haakon-wium-lie]

Yes, that's how the spec is written: the style sheet specifies an optimal line length, but it will be adjusted somewhat to fill the whitespace.