Accessibility Principle: Provide text alternatives for non-text content.

The alt attribute is used to specify a short text alternative when using the img element.

Accessibility essentials:

• W3C Web Accessibility Initiative
  The Web Accessibility Initiative (WAI) develops strategies, guidelines, and resources to help make the Web accessible to people with disabilities.
• Accessibility - W3C
  The first place to start for a short introduction to Web accessibility.
• Web Content Accessibility Guidelines (WCAG) 2.0 at a Glance
  This is a summary of the WCAG 2.0 technical specification.

The power of the Web is in its universality.
Access by everyone regardless of disability is an essential aspect.
Tim Berners-Lee, W3C Director and inventor of the World Wide Web