CSS usage metrics (chromestatus.com)

Could be mostly pages without CSS. But layouts that don't need to restrict the width in any way do exist.

I thought the same exact thing. I've seen some newer CSS frameworks using stuff like min-width (I think for responsive reasons), so maybe that accounts for the 6%? Just a theory, though.

Is actually pretty easy to design without width; you can use max-width OR min-width in a per case basis, you can use margins (a left margin of 10% and a right one of 10% means your width will be roughly 80%) or padding in a similar fashion.