Information Technology Systems & Services
Use Relative CSS Units
To ensure that the user of your web pages can set the size of text to suit their own preferences you should either not set a text size at all, or control text sizes with relative Cascading Style Sheets (CSS) units.
Relative sizing is scalable. It is the opposite of absolute sizing. For usability and accessibility it is better to use relative rather than absolute sized units. Use relative units such as percent or ems. Relative sizing is scalable. It is the opposite of absolute sizing. For usability and accessibility it is better to use relative rather than absolute sized units. Use relative units such as percent or ems. I use em units because it allows users to resize their text where pixel sizes, in IE PC, do not. Putting 100% in the body selector kills an IE bug. After that you can use em. (1em is equal to 100%.) By setting the sizes of the divs in ems as well as the text sizes, the whole design scales up if the user bumps their text size up a bit. Many low-vision users make use of devices to enlarge content displayed on their monitors. By using relative units, these users are able to change the size of the text without much difficulty.
Points are for print. Don't use them for screen output. Em is a best practice for relative fonts. In CSS an em is what is referred to as a relative unit, just like percentages. The W3C explains em units this way:
" 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.)" 1
The reason for avoiding fixed font sizes (i.e., any font-size value in mm, cm, in, pt, etc.) is that eyesight varies, and so do the external conditions for viewing. The size should be what the user has chosen, or something in reasonable proportion to that.
For more text sizing information visit: