I can't say for sure, as SEO is never one of my priorities (subject to change in the future). However, accessibility is always very important.

I'm told (by people like you) that they are synonymous, and the sites I write to tend to rank high, but they do so for relatively narrow domains, and it takes a while for them to get there. I'm not sure if it is the accessibility, or Big Fish in Little Pond.

Think about it ... Googlebot is blind. The layouts and labels that help a page make good sense when accessed with an audible browser reader will also help it make sense to Googlebot. Paying attention to accessibility techniques will boost your SEO even if you've never heard of search engines.

However, accessibility in itself doesn't cover every base that needs to be covered for effective SEO. A page could pass every accessibility test with flying colors but be aiming at an unproductive search phrase (or not aiming at anything in particular).

This is where good SEO judgement comes in. It takes good keyword research skills and a good understanding of the user mindset to discern the most effective search phrase(s) to aim for.

It also takes a good writer to create headlines and body copy that flow well for humans as well as pleasing the spiders.

I don't know if I agree that they are synonomous... I would be more inclinded to say that usability and SEO should be viewed as complementary. One might see accessibility and SEO as synonomous. On a well SEO'd site I believe you'll get accessibility by default in many cases - there will be a lot less text buried in images, all images will have ALT text associated with them, etc. making the site more accessible to readers and such.

A site should be both usable AND well SEO'd IMO. When an issue arises and I have to make a choice between usability and SEO, I typically error on the side of being usable first. So while SEO is very important, IMO usability is more important.

Having a well SEO'd site may bring lots of visitors; but if the site is not user friendly, the users will not stick around for long or will be unable to find what they seek.

Having a site that is very usable typically doesn't do you a lot of good if people can't find it through the engines unless you are getting all of your traffic through pay-per-click.

So I think you have to aim for striking a good balance between usability and SEO. If those two are done well, accessibility will to a great extent take care of itself.

[edited by: ZydoSEO at 6:46 pm (utc) on Jan. 1, 2008]

Accessibility, Usability, and SEO are three different things, each with separate goals:

• Accessibility's goal is for users to be able to access the content via multiple platforms, but says nothing about how easy it is to access, nor if search engines index the pages.
  
• Usability's goal if for the site to be easy to use, but doesn't preclude limiting access to but a single platform, nor does it speak of how to get to the site.
  
• SEO's goal is only to get you to the site, but what happens next is irrelevant.
  

That said, it takes effort to get them to work against oneanother, and when done correctly, Accessibility, Usability, and SEO all complement eachother:

• Accessible content forms a good basis for usability for it's hard to use what you can't access. It is also good for SEO because if search engines can access the content, they can index and weigh it.
  
• A useable site will include usefull content, which means more content for a search engine to index, and therefor, weigh more accurately.
  
• One of the most effective methods of SEO is to provide lots of content for the search engine, and the content which is best for the spider is usually also very good for people. And if a search engine can access it, people probably can, too.
  

It should be noted, just because they each enhance oneanother, with little effort, "little effort" isn't the same as "no effort." Furthermore, to get the most out of each of the three usually takes a lot of effort.

If I were to know nothing about SEO, never heard of it, and I needed to build a site that appealed to a certain audience. If I followed the guidelines above at the WAI, how do you think I would do out of the gate?

It would be a very good start, but races aren't won by starts allone.

Would I have covered 100% of my onsite SEO just by making sure that my users were able to access and use everything?

No; not if the desired search terms are even slightly competitive. Good SEO is a continuous process of adding more and more fresh content. Yes, fresh content, as search engines tend to favor newer content over older content.

If your site is not SEO then you should work on it regardless, and think of it as a usability issue. One of the items I test for when doing my analysis is your site name, and a few keywords show up in the top 40 search results. Here is an example why you site needs to at least do this.

Imagine you have a store that sells boating supplies. You tell a customer in your store about your website, and that they can even buy parts online there. They go home, forget the domain name, but remember your business name. They type it into Google, and to no luck can not find your store. You just failed usability test #1, and they just found a new supplier other than you.

Not sure if I am qualified to answer this but I do know that when I realised that I should be putting alt tags on my images that alt text seemed to help my pages search for those terms. Only on standard search though, the ranking on image search is dire.