[EditHint: Moving content to UniversalScreenUnit]

(I know I wrote about this somewhere else on WikiWiki, but couldn't refind it.)

In writing HMTL output, or any GUI design, I find it difficult to specify a sizing unit that is universal and lasting. Often "pixel" is used in practice, but I suspect that actual addressable monitor pixels will keep increasing over time such that relying on "pixel" will make stuff shrink too small. Text will just grow larger relative to pixel and addressable monitor resolution will be so small aliasing problems will become less of any issue. Aliasing problems, and related blurring, have been the biggest stumbling block against abandoning pixel-matching fonts. Addressable monitor resolution is getting near the threshold where aliasing and blurring are no longer bottlenecks.

Thus, we need something more "stable" than "pixel".

On a smaller scale, "points" is often used because it's based on the display of fonts, whose size should stay the same over the time because its limited by human eyes, not technology.

However, points don't work well on a larger scale since they are designed with typography in mind. For example, HTML tables. 

Percent of screen is one possibility, but screens are also growing larger and/or longer, and are sometimes set up to wrap to a second screen.

After thinking pondering this for a while, I've come up with a possible solution. I call it "'''Normal Paragraph Width'''" (NPI). This is the size of a "friendly" paragraph. It corresponds to roughly 10 to 15 average words long and is based on the concept that readability is considered maximized around this size. Publication experts sometimes talk about this width limit also. Short paragraphs and long paragraphs are both hard on the eyes for extended reading time. 

Thus, NPI is based on human WetWare, not so much on technology, and thus will generally be stable, outside of technology change.

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Isn't "Normal Paragraph Width" entirely dependent on font size?  Or is it meant to be dependent on font size?

Why is "Normal Paragraph Width" abbreviated "NPI"?

You say points don't work well on a larger scale, which is true, but there are picas. A pica is 12 points.

We also have handy, pre-existing units like millimetres, centimetres, decimetres, metres, inches, feet, etc.
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See also: TheProblemWithIcons