The problem with fonts.

Nobody knows what a normal sized font is

12 Point Default 14 Point

IE has a "Default" font size of 14, NS has a "Default" of 12. Mozilla under Windows seems to take the IE approach for feature parity, and under Linux takes the NS approach for the same reason.

The same sized font isn't the same size

Serif Sans Serif Fantasy Monospace Verdana Arial Times new Roman Comic Sans MS Cursive

The ratio of the height of the "x" and the font size makes some fonts appear different sizes even if they are identical. Antialiasing can make this appear even worse.

Points vs Em vs Pixels

Name What it's supposed to mean What it actually means What the W3C think it means
pt 1/72nd of an Inch Since most OS's assume 72DPI, so this is usually the same as a pixel
In theory something specified in pt's will be the same size irrespective of resolution, in practise who knows?
1/72nd of an Inch
px One pixel One pixel For reading at arm's length, 1px thus corresponds to about 0.28 mm (1/90 inch).
- I kid you not.
em The width of an "m" The default font size The 'em' unit is equal to the computed value of the 'font-size' property of the element on which it is used. The exception is when 'em' occurs in the value of the 'font-size' property itself, in which case it refers to the font size of the parent element. It may be used for vertical or horizontal measurement. (This unit is also sometimes called the quad-width in typographic texts.)
ex The width of an "x" Half of 1 em
in Inches 2.54cm
cm Centimeters
mm Millimeters
pc Picas 12 Points

So there you have it. One pixel == 1/90th of an inch, One point == One pixel, One point == 1/72nd of an Inch. and therefore an inch is only 80% of what it once was.

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