When an appropriate markup language exists, use markup rather than graphics to convey information. For example, use MathML to mark up mathematical equations instead of using graphics to represent symbols.
The Example
x2 α=0.4
The HTML
<math>
<msup><mi>x</mi><mn>2</mn></msup>
</math>
<math type="text/mathml-rendererB">
<mi>α</mi><mo>=</mo><mn>0.4</mn>
</math>
Many users do not use equipment that is capable of running scripts, applets or other programmatic objects. For example, they may have old or non-mainstream hardware and software that does not support the scripting languages or supports only earlier versions. They may be working in an environment where scripts and applets are blocked by a firewall for security reasons. They may have a physical impairment that means they are unable to use a mouse and cannot therefore interact with scripts that require mouse input. Or they may be using a form of assistive technology, such as a screen reader, which cannot access the contents of the script, applet or object. Other users may have turned off support for script handling for reasons of speed or security.
More information on MathML - Mathematical Markup Language 1.01 Specification.
View WAI Checkpoint 3.1 - When an appropriate markup language exists, use markup rather than images to convey information.