POR - we used definition list mark-up on our site, and pulled in a lot of define: searches (accidentally, really). Since then, the world's major financial institutions and dictionaries appear to have caught up, and we no longer rank on all of the phrases - external links count for more than on-page mark-up.
As a quick addition on definition lists, you can use multiple definitions for a single term, e.g.
<dd>Big brown animal. Fierce.</dd>
<dd>Short for Robert</dd>
And so on.
Re: <abbr> and <acronym> - I find it easiest to think of it as a pronunciation guide.
<abbr> means that you sound out each letter if it's IN CAPS, or read as a word if not. <acronym> is where you read it as a word even if it's IN CAPS. That said, this is a personal view, and I haven't checked out how JAWS is treating them, for example. If anyone cares, I'll go do just that.
Re: 'User Agent' - this is not meant to be a cryptic reference to search engine bots. A user agent is any software that retrieves and presents Web content for users. Examples are Web browsers, media players, plug-ins, and other programs — including assistive technologies — that help in retrieving, rendering, and interacting with Web content. You can consider Googlebot as behaving very like a user agent... but it isn't a user agent in the strict sense.
I think you may find that the whitespace is being caused by the line-break between the label and the input element. Think of it as behaving like any other inline element (e.g. <strong>) - a line break after it becomes a space when rendered. I would strongly recommend using the first method, i.e. label not containing the input element.
re:headings (hmm... I wonder if you can put headings in these posts. Never mind. Sorry if this is getting long!)
Yes, use headings to give structure to the page, but remember that a h3 following a h1 should really be avoided, as it skips a level. h2 after a h4 would be fine though. A subheading should be exactly that, a subsection of the topic of the parent heading.
I would avoid using headings for navigation on the whole. The headings should be about the specific content of the page, not the architecture of the site overall. Keep the navigation in an unordered list.
Absolutely right about concision - would also recommend 'front-loading' of headings (and title elements. And paragraphs) - the most relevant text comes first, followed by qualifiers, followed by supporting information.
POR, I shall pass that back to the working group. They will be delighted to hear it! We like to hear criticisms too - as mentioned in my previous comment, please do comment wherever you see any entry that isn't clear.
There are still many arguments on this. If you are designing slickly, you would only ever use an image where it had meaning, and so it would have something in the alt atribute... andthing that really is purely meaningless decoration should be provided as a background image via the css. That said, when you're wrestling with a recalcitrant CMS, an img element with a null alt attribute can be the least worst option. Don;t forget that not all screenreader users are blind - if you have low vision, you may be really interested in knowing what the orange blur is at top right.
re: nested lists.
Semantically, nested lists are great. Trying to apply CSS to them that allows for liquid layouts and a set of tabbed horizontal nav bars can be humungously frustrating. If anyone manages a good implementation, please let me know!
N.b. - blimey, we could do with sprucing up the accessibility of webmasterworld, couldn't we. Ick.
[edited by: tedster at 10:25 pm (utc) on Feb. 18, 2009]