Can we use any extension in LONGDESC or only .html is allowed by W3C?

Can we use any extension in or only HTML is allowed by W3C? and how LONGDESC link should be open , only as new windows or can we open Longdesc url in popup or lightbox?

Does LONGDESC have a开发者_如何学Pythonny benefit for Site SEO?

Can we use any extension in or only HTML is allowed by W3C?

The specification doesn't mandate what type of resource should be at the other end of the URI, but an HTML document is most sensible

and how LONGDESC link should be open , only as new windows or can we open Longdesc url in popup or lightbox?

I'd set up a longdesc and then leave it alone. I wouldn't go near it with JavaScript. New windows and lightboxes are accessibility barriers and should be avoided at the best of times — that goes double when the resource is designed primarily for the benefit of those needing assistive technology.

Does LONGDESC have any benefit for Site SEO?

Possibly. Search engines don't publicize the details of their algorithms. As a rule of thumb "Forget about SEO. Design for users and trust search engines to do their job.". Again, this goes double when implementing an accessibility feature.

The "extension" (i.e. optional text after the final dot) is not prescribed by W3C. It can be anything, or even absent.

Your web-server may need to be convinced to provide the right MIME types. Many web-servers choose MIME types by looking at the extension.

I believe that any extension is acceptable. The spec makes no mention of extensions or mime types. As for using it in a popup or lightbox, that's easy: just use AJAX to load the URL and add the contents to your popup. In jQuery, it's be something like this:

$('a[longdesc]').click(function() {
    $('#popup').load($(this).attr('longdesc'));
});

As noted on stuffandnonsense:

Sadly the longdesc attribute is invisible to those not using screen-readers and goes unsupported by some browsers.

There's no citation on that, and there's no mention of which browsers do or do not support it, but I think it's safe to say that it's not going to help your SEO too much. Take a look at that site for some good alternatives which will work for all visitors.

For best SEO, make the longdesc point to an anchor on the same page. If necessary, move the block containing the description off-screen, so that it is not visible to sighted users.

Also, consider using aria-describedby instead of longdesc.