From HTML WG Wiki

Conclusion

In conclusion, longdesc should be included in HTML5 as an allowed attribute on images because it:

• Provides a solution to specific use cases. It solves real problems.
• Provides an accommodation to blind users and avoids the flawed "one size fits all" solution.
• Affords authors the native capability to provide information that is essential for blind users but would be redundant for sighted users and unacceptable to visual designers' aesthetics.
• Does not force a visual encumbrance or default visual indicator on sighted users.
• Programmatically ties a description to an image.
• Provides a method for the long description to be programmatically determinable when an image already has a link which is mapped to go to another document or a larger image.
• Provides easy reuse of the targeted description from multiple sources (i.e. ability for an image to appear on multiple documents throughout a site or throughout multiple sites or from an HTML email while at the same time linking to one longdesc document).
• Permits pointing to external sources that allow the full use of HTML as well as other markup languages, such as MathML or SVG.
• Allows screen reader users freedom of choice. The description is not forced upon them whether they want it or not. They can interact with it at their own will. With other solutions like aria-describedby this is not possible.
• Enables AT users to use reading keys to control how the information is provided to them. With other solutions like aria-describedby this is not possible.
• Allows structured content in its target.
• Strengthens HTML5's capacity to express rich semantics. A longdesc's purpose is to provide a long description.
• Is backwards compatible.
• Provides ease of use. It is a simple link.
• Saves bandwidth, for those users who don't need a long description.
• Is recognized by existing authoring tools, user agents, assistive technologies, educational material, etc. Has a critical support base that has taken a decade to build and would probably take another to rebuild with something else. Something new would setback progress.
• Unlike options such as using <object data=image><markup/></object> (or <canvas src>), longdesc does not require any change or "messing" with the ARIA role or of the <img> element.
• Is in current use on numerous company, organizational, governmental, educational, and personal sites throughout the world.
• Has some native browser support which is expected to continue as part of support for HTML4x. Extensions also exist.
• Is implemented in numerous authoring tools.
• Is implemented in popular assistive technology.
• Is in numerous tutorials and documentation throughout the world.
• Is in numerous guidelines throughout the world.
• Is law in various countries.
• Is policy in many organizations and companies in throughout the world.
• Is a standard in numerous organizations and companies.
• Provides authors a unique tool to use when needed. Stripping it from them is a disservice.
• Provides native semantics.
• Protocols and Formats Working Group (PFWG) "likes the idea of having built in semantics in HTML and in particular would prefer to have common document elements."
• Provides a simple, built-in solution rather than work-around hacks. It avoids needless complexity.
• It would allow longdesc to be improved. (Bug 10015: longdesc URL checking, Bug 10019: Native user agent support for exposing longdesc to all users).
  • Exposition of longdesc would help authors debug. Reference: Adam Sampson.
• It would possibly allow longdesc to be used on other elements that need descriptions.