[perceived background image resolution and page zoom]

I don't believe there is anything you can do about this at this point. Zooming is entirely within the user's control, so it isn't really something that you can anticipate or adapt for as far as I know.

 

It's obvious to me at least that the graphics on some sites looks better when zoomed than others. That's preprocessing and intelligent choices I'm sure. I'm interested in strategies for this.

[perceived background image resolution and page zoom]

Considering page zoom which enlarges also all graphics and not just text, what techniques — graphical preprocessing, CSS, JS or combinations — do you use in order to achieve acceptable "zoomability" for image background?