CSS px are not intended to be mapped 1:1 with screen pixels. They are actually defined as an angular measurement: http://inamidst.com/stuff/notes/csspx
Web browsers running on modern hi-res screens need to be calibrated by "zooming in". This happens automatically with Apple Retina Display. For images to fully utilize hi-res screens, image dimensions need to be twice the CSS dimensions.
Interestingly, the JPEG quality setting of images on hi-res screens can be reduced dramatically without compression artefacts becoming visible, meaning that filesizes are not much larger; it is even possible for the filesize of hi-res images to be smaller: http://blog.netvlies.nl/design-interactie/retina-revolution/
jQuery AJAX Load is an helper module for easy loading any content using build in jQuery load function.
Just enable the module and add class .jquey_ajax_load to any link, URL content (without other regions) will be loaded on #jquery_ajax_load_target element. Those classes and id could be personalized to your needs. Target element coould be set on a per-link basis using the data-target attribute to define a valid jQuery ID.
Optionally css and js files could be loaded or not. Loading those files has an effect on performance, so load those only if needed.
It comes with a set of modules that uses the functionality for: