CSS Font Size Specifics

Question

I have a couple of specific questions about font size:

1. Is pt (I.E. 12pt) standardized? I think I read somewhere that pt = 1/72 of an inch. However, the size

Answer 1

1)

In print css, a pt is exactly 1/72 of an inch.

In screen css everything is defined in terms of pixels, and a pt is defined as 1 1/3 pixel. But how large this is in physical measurements like inch depend on screen DPI and vary greatly between devices. For this reason, pt (and other physical measurements like in, cm etc.) is not that useful in screen css.

2)

Font-size refers the the height of the "em-box" - a bit simplified this is the rectangle which can contain all letters in the font including ascenders and descenders. Individual letters will of course often take up less space than the whole em-box. For example a lower-case x will have space above and below, while a lower-case j might fill the whole height of the em-box.

Font-size therefore indicates how much vertical space is required by a line of text, but the dimensions of individual letters and glyphs will vary relative to others depending on the letter and the design of the font in use.

Furthermore some fonts may have letters which extend ("bleed") even outside the em box, which is also a stylistic choice by the designers of the font. E.g. the letter "Å" will often bleed above the em-box.