What is a pixel (px) in CSS?

Question

I am reading an ASP.NET book and it refers to CSS files and talks about pixels. But I never understood it from a resolution, layout, etc. point of view. For example, what does t

Answer 1

This is a little beyond where you might be at the moment, but a CSS pixel is not necessarily exactly the same size as a single pixel on your display. According to the spec:

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 the visual angle of one pixel on a device with a pixel density of 96dpi and a distance from the reader of an arm's length.

So if you have one of those incredibly expensive extra-high-resolution displays that doesn't count as “typical”, the browser and/or OS may choose to redefine what a “pixel” is.

The useful definition for a ‘px’ as far as CSS authoring is concerned is: a ‘px’ is the quantity of length equal to the pixel in an unscaled HTML or CSS ‘background-image’.