> This page just needs max-width set somewhere on the text container.
So, set max-width on body in your user.css?
Personally I've long since stopped reading the web in "full screen" -- I'm more than a little surprised that people still do that. Assuming you're reading text, why would you want to have to move your eyes across the entire screen to take it in? Much better to have an appropriately sized window IMNHO.
body { -webkit-column-count: 3; } // or -moz- or no prefix or whateverMight split out the header separately.
In mobile, rotate the device to vertical.
It's better for the user if a site has good readability without them having to do anything.
Impossible. Because you cannot assume user's taste.
Also, if you think readability is not a taste, and you can make a mostly ideal configuration which can be applied to all the users, why don't you let the browser vendors to do that?
If browsers are shipped with ideal configuration by default, then now the only obstacle to prevent ideal readability is site-prodivded styling.
Actually Safari is trying to do this via Reader button, and shows always great result if it is available.
It's not really possible to enforce line-length with a user stylesheet (increasing the point size doesn't solve the problem, it just balloons the letters up to goofy proportions). This page just needs max-width set somewhere on the text container.