Semantically Incorrect

Semantic elements and attributes provide a higher level of communication. The semantics of summary/details does not provide an explicit and strong semantic for an image long description. This technique provides incorrect semantics for long descriptions of images. The HTML specification states,

The summary element represents a summary, caption, or legend for the rest of the contents of the summary element's parent details element, if any.

An img element isn't a summary, caption, or legend. An img element is an image. An image and a long description are peers: the same thing expressed in two different mediums.

Forces a Visual Encumbrance

The summary/details technique would force a visual encumbrance on sighted users unless additional code (in the form of CSS or hidden) were added in order to hide the visual indicator. Please refer to the Cascading Style Sheets and hidden documents for details of why using these techniques is a bad idea.

Lacks Ability to Provide Description in a Separate Document

The summary/details technique lacks the key functional requirement of providing a long description in a separate document.

Lacks Assistive Technology Support

summary/details is not accessibility supported semantics wise. HTML5 accessibility states that that this technique,

Currently provides the same amount of semantic information to AT as a div element.

Lacks Backwards Compatibility

The summary/details technique for long descriptions lacks backwards compatibility. It is not:

For further rationale please consult the backwards compatibility document.

Retrofitting Problem

Any proposed solution should be easy for authors who are already publishing content with longdesc to retrofit their existing pages. summary/details possesses a radically different authoring pattern than longdesc. Because of this as well as its on-page limitations, retrofitting with summary/details would be difficult, labor intensive, and error prone.

No retrofitting is required for longdesc as it is an existing HTML feature not a new one.

No Examples in the Wild

No examples in the wild of accessible long text alternatives with summary/details have been presented for any of the use cases. No evidence exists that authors have or will use it for accessible long text alternatives.

No Evidence of Improvement

No evidence has been presented that summary/details produces more accessible content for long descriptions than longdesc or that more authors would use it correctly.