A very good slideshow from Stephanie Rieger. She suggests using a small number of stylesheets and media queries targeting max-device-width (rather than device-pixel-ratio). then having in-line media queries in individual stylesheet like
Because Android hi-DPI devices return window.devicePixelRatio > 1, to correctly detect Retina make sure the value is >= 2.
Also worth using email@example.com naming convention because it is the standard used in native iOS development.