Nobody can understand what the hell a ""Web unit"" is. In the following explanation -
A Web unit conforms to WCAG 2.0 at a given conformance level only if all content provided by that Web unit (including any secondary resources that are rendered as part of the Web unit) conforms at that level.
- what happens if I have a page full of thumbnail images, each with correct alt text as required and each of which links to an image file of a larger version of the picture? Since the image by itself has no HTML or other markup, it's impossible to write an alt text for it. Is this not a ""secondary resource""? If it isn't, does it not then constitute a ""Web unit"" unto itself? Since Web units that are simple image files cannot be made accessible, doesn't WCAG 2 essentially ban freestanding image files?
(We are later told that linking to nonconforming content ""is not prohibited"" - gee, thanks - but only if ""the content itself is [INS: [not] :INS] a Web unit within the set of URIs to which the conformance claim applies."" Hence if my freestanding image is still hosted on my site, I have to make it comply with my conformance claim, which at the very least requires a text equivalent, in turn meaning I have to wrap the image file in HTML. But by the time you the site visitor have selected and loaded that expanded image, you will already have had a chance to read the alt text on the thumbnail image.)
We have revised the guidelines and eliminated the word "Web unit" in favor of "Web page." We have defined "Web page" (see http://www.w3.org/TR/2007/WD-WCAG20-20070517/#webpagedef ):
a resource that is referenced by a URI and is not embedded in another resource, plus any other resources that are used in the rendering or intended to be rendered together with it
Note: Although any "other resources" would be rendered together with the primary resource, they would not necessarily be rendered simultaneously with each other.
Example 1: When you enter http://shopping.example.com/ in your browser you enter a movie-like interactive shopping environment where you visually move about a store dragging products off of the shelves around you into a visual shopping cart in front of you. Clicking on a product causes it to be demonstrated with a specification sheet floating alongside.
Example 2: A Web resource including all embedded images and media.
Example 4: A customizable portal site, where users can choose content to display from a set of different content modules.
In the situation that you describe, the freestanding images would constitute separate Web pages and would need to conform to WCAG or not be included in a claim. Putting it in an HTML page would be an easy way to make it accessible if that was desired.