Stating opinions like facts, the standard units should be: 1) Arcminutes at typical view distance. The sun is 32 arcminutes and a css pixel is 1.278 arcminutes

Stating opinions like facts, the standard units should be: 1) Arcminutes at typical view distance. The sun is 32 arcminutes and a css pixel is 1.278 arcminutes. This is the primary unit for measuring content. 2) Percent. This is the primary unit for measuring layout. The number of arcminutes per percent varies from screen to screen in the same way that screen sizes vary, and can be dealt with using the same responsive design tools. 3) Pixels. A pixel is the smallest size that can be drawn in this medium (it could be an actual pixel or an inkjet dot). A 1px line possibly useful as the finest possible thickness, but otherwise you should only use this if you have a specific reason, and mixing it with other units invites madness. 4) Meters. Literal physical size. Like pixels, there are specific reasons to use this, but even fewer of them, and mixing it with other units invites madness.