Text on a web page, including text-based controls such as navigation links, should have the ability to be scaled so that people with mild disabilities (or even just folks who use reading glasses) can read the text directly without resorting to the use of a screen magnifier or other assistive technology.
Most browsers these days have the ability to scale text content built into them. If the web page creator, however, designates exact sizes for text, the browser will not be able to enlarge the text at the reader’s command. Most web designers have become accustomed to specifying text in a web page in terms of pixel size. For example, here we have specified a fixed font size in the code of a web page:
<font size="3">Here is some text.</font>
The browser reads the 3 pixel font size and will hold the text at that fixed size, regardless of efforts made by the reader to change it within the browser software. If we make a minor modification:
<font-size: 100%>This is some text.</font>
The font will be displayed at whatever is a 100% size in the browser of the reader’s choice. The reader can then choose to zoom in or out using the built-in capabilities of the browser.
For more information and tips for implementing on your own website, visit: http://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-scale.html.