css unit: em

http://www.w3.org/TR/CSS2/syndata.html#length-units

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.)

http://css-tricks.com/css-font-size/

Here’s the scoop: 1em is equal to the current font-size of the element in question. If you haven’t set font size anywhere on the page, then it would be the browser default, which is probably 16px. So by default 1em = 16px. If you were to go and set a font-size of 20px on your body, then 1em = 20px.

Things start to get slightly more complicated with em’s when we start setting up more complex font sizing. Say we need a header with a larger font-size, so we set h1 { font-size: 2em; } That “2″ is essentially a multiplier of the current em value. So if the current em size is 16px, that header tag is going to turn out to be 32px. That math works out cleanly, but you can imagine that it often doesn’t and rounding needs to take place.

The most popular method in working with em values is to set the font-size on the body to 62.5%. Because the default browser font-size is 16px, this makes it 10px (without hard-setting it to 10px, which wouldn’t cascade). Using 10 as a multiplier is much easier than using 16. This way, you need a font size of 18px? Use font-size: 1.8em.

So why both with all this em business when it’s just an abstraction of using pixel values anyway? Three possible reasons:

1. The ARE resizeable in IE 6
2. The relationship to other sizes (elastic width sites)
3. Em’s cascade like a mo-fo

Em’s aren’t just for fonts, it’s a unit of measure that you can use for any other length (height, width, etc). Elastic width sites use em values for everything, which essentially makes the site “zoomable”, meaning that when you bump the font-size up everything bumps up all the way down to the width of the site. Em’s have a direct relationship to each other in this way. If you have a box that is 10em in height, and a font inside that is 1em in size, it will take up exactly 1/10 the height of that box. That exact proportional relationship makes em values a powerful web design technique.

There is one potential concern with em’s, with regards to #3. They do indeed cascade. Every em value is relative to its parents value. If you are using em’s as a straight substitution for pixel values, this can cause problems. For example, you might set both your “p” (paragraph) and “li” (list item) font-sizes to be 1.2em. Looks great for you today, but tomorrow some content is published to the site that has a paragraph inside a list item. Those two values will cascade (1.2 x 1.2) and that list item will be bigger in font-size than any of the others. No way around that, other than removing the tag.