New websites should be designed and developed for HiDPI screens anyway, and the retina screens of the MBPs and iMac hugely help with this.
A 900px image should be designed and saved at 1800px, then scaled down using CSS or a more complex adaptive image solution. This ensures HiDPI screens show super crisp images, and regular screens work as normal.
I miss my 5K for this very reason. It was really handy.