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Comment by wmf | Hacker News Reader

Note that a px is not the same thing as a pixel. On a 250 PPI display 1 px probably equals 2 pixels.

http://webkit.org/blog/55/high-dpi-web-sites/

This just makes me angry. The whole point of being able to specify in px was to have something that is actually specific to the device's pixels---otherwise you'd use points or inches or cm---for those rare instances where the pixel count is actually relevant. Of course, it got overused, but the answer should not be to just redefine "pixel" as "something a bit smaller than a point" (except in a bit more detail)! Argh.

Actually, rescaling pixels is part of the CSS specification:

     If the pixel density of the output device is very 
     different from that of a typical computer display, the 
     user agent should rescale pixel values. [...]
     It is recommended that the reference pixel be [...] one 
     pixel on a device with a pixel density of 96dpi

http://www.w3.org/TR/CSS21/syndata.html#length-units

I may be reading it wrong, but it sounds like a px is a pixel—until you zoom in. If you define a web page’s default font size as 12px, and it stayed at that no matter how much you zoomed in, it wouldn’t be much of a zoom feature, would it?

That would be reasonable, but the CSS standard says:

"If the pixel density of the output device is very different from that of a typical computer display, the user agent should rescale pixel values."

It then goes on to define a "reference pixel" in terms of arclength subtended within the user's field of vision. On reflection, I suppose this is sufficiently different from pt/in/cm (which measure absolute length) to merit its own separate measure; I just wish they hadn't used the term "pixel" for it. Because "pixel" already means something else and the arclength subtended by a pixel in a "typical computer display" has changed considerably over the last twenty years.

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