Can longdesc be used without meaningful alt text?

I'm attempting to write good fallback text content for a webcomic. Naturally, there is a huge amount of actual text locked in the image, and plenty of descriptions/actions/expressions that could also be described. Having longdesc="#transcript" seems like the perfect use case, and comes with benefits for searching and automatic translation.

But what do I do with the alt? I've checked the official specs, and dug around in WebAIM and similar sites, but I've never seen a use case for having longdesc supplant alt. This makes sense for the usual applications (overview a chart in the alt text, link to a full breakdown elsewhere), but it seems like any alternative text I could offer for a comic would be redundant and miss out on the rich markup provided by the long description.

Here are some possibilities:

<img alt="" longdesc="#transcript" />
<img alt="[transcript text stripped of HTML and made attribute-safe]" longdesc="#transcript" />
<img alt="[Summary of comic contents... Which can get iffy, like this: 'Garfield talks about being fat. Punchline: he's fat.']" longdesc="#transcript" />
<img alt="[apologize profusely to screen reader users]" longdesc="#transcript" />

None of these seem ideal for various reasons, whether that be repeated content, no longdesc support, or me annoying Assistive Technology users. Without a sound declaration from folks who have thought about and dealt with this stuff way more than I have, I'm at a loss.

Solution

The alt attribute is required for accessibility, and it is even formally required by the HTML 4.01 specification. The use of the longdesc attribute, even if it were implemented, would not make the alt attribute unnecessary. The description of img in HTML 4.01 shows the following example:

<IMG src="sitemap.gif"
     alt="HP Labs Site Map"
     longdesc="sitemap.html">

It adds: “The alt attribute provides a short description of the image. This should be sufficient to allow users to decide whether they want to follow the link given by the longdesc attribute to the longer description, here "sitemap.html".” (Whether this would be adequate even if longdesc were supported is a different matter.)

Due to lack of support, longdesc has remained useless. In order to refer to a long description, you need to use a normal link near the image. This lets anyone (even people who can see the image but may need some explanation) access the description.