I think there are a lot of unanswered questions about @fullsize and
the current proposal will be met somewhat skeptically with
implementors of HTML user-agents. It might help first of all to think
of all the possible use cases for the feature. Think in broad terms,
consider how @fullsize would work on the internet & intranet, and ask
yourself if it would impact legacy markup in anyway. It helps to put
on your "archeologist" cap: dig through forums, email lists, bug
databases, etc.

General questions:

1. Are the similar ideas in the wild right now (rhetorical)? Maybe a
microformat that does the same thing? A javascript library? How do
they work, what are the pro & cons. Document your findings.

2. Are their parallels to current or previous features? What gaps does
@fullsize fill in? Consider previous attributes like @lowsrc. What
we're the problems? Netscape 4, IE 4, and IE 5 supported @lowsrc but
more recent browsers aren't really doing anything with it. As far as I
know, @lowsrc never made it into an HTML working draft or
recommendation... why?

After thinking about the above, you might be ready to tackle the
specific implementation questions:

1. What happens when the ratio of the @src and @fullsize image
resources aren't the same? What role does @width and @size play, if
any?

3. How does @fullsize interact with CSS? If @fullsize takes multiple
values, how does that work in CSS?

4. Are the resources in @fullsize pre-fetched? Left up to the user-
agent? End-user?

5. What is the DOM interface for @fullsize? If multiple resources are
allowed how does pushing and popping new ones into the DOM work? How
would DOM Events apply to @fullsize? and so on...

6. What happens when an @fullsize resource request fails? (You can
examine the spec's language for @src for the most part... but consider
what makes @fullsize different)

7. What are the accessibility issues for @fullsize? For instance, do
you need multiple @alt values?

8. As mentioned in IRC (#whatwg), is doing this going to alter the
semantics of <img>?

I don't know the answer to most of the questions, they aren't being
asked with any agenda in mind... but I can tell you that if @fullsize
made it out of the "we have sufficient use-cases & evidence to support
this type of feature" stage, then people are going to ask the nitty
gritty questions.

Keep in mind that @fullsize (a new attribute on <img>) might be the
wrong solution all together -- but, hopefully, in doing the work to
explore @fullsize you'll figure out the potential solutions... there's
usually multiple and they always have pros & cons.

Personally, I think this is an area best left to web authors innovate
in. I'm inclined to think the existing markup pattern (anchor pointing
to a full size image, with a nested image element representing the
thumbnail) is suitable for a user-agent/plugin/javascript library to
do something clever with if they wanted to. Even if implemented, the
user-agents would likely provide a basic UI and I don't think the
experience will be rich enough. (I base that on the *many*
implementations of a thumbnails, galleries, etc on I've used on the
web... they're all very different .. many audiences in mind..)

Cheers,
-s