Several absolute font-sizing keywords are available. The font size is determined from a browser preset and the element will not inherit its parent’s size.
Although most browsers support these keywords, the exact sizes will differ. They are a fairly crude method of font sizing and are generally avoided by most developers.
Two relative font-sizing keywords are available. The font is sized according to its parent element:
For example, if the parent has a font size of ‘medium’, a value of ‘larger’ will set the element to ‘large’. Other font units are normally altered by a factor of around 1.2 but, again, there is no standard and browser results will differ.
font-size property can be assigned an absolute length:
In general, there are issues with all these measurement units. Millimeters, centimeters and inches are inaccurate for a screen-based medium. Points and picas are unreliable since systems can use different dpi settings. Pixel appears to be the most suitable, but it can lead to accessibility issues because the text cannot be resized in IE.
font-size property can be assigned a unit that it relative to its parent’s font size:
Few developers use ‘ex’, but it can be useful in some situations where you need fine-grained font sizes, e.g. 1ex rather than 0.525em.
Percentage and ‘em’ sizes are equivalent, e.g. 50% = 0.5em, 100% = 1em, 120% = 1.2em, etc. Some browsers exhibit subtle differences but it’s rarely a major problem. If you want to save every byte, you could choose the shortest definition, i.e. 50% is shorter than 0.5em and 1em is shorter than 100%.