I think it's important not to get hung up on the definition of usable and accessible.
Say you start with a hypothesis: You want to create a website that is usable by anyone. Literally. Anyone.
Immediately you think 'well that's just not possible, surely' - not anyone! We can't make our banking website usable by a 2 year old child with severe visual impairment or blindness." You can't help but try and draw a line somewhere. A line that is comfortable for you, your project, your business, your budget. With the best will in the world, it can't be done for everyone, and there are laws of diminishing returns.
So a practical starting point is to start thinking about who your audience is, and make it usable for them.
By that, of course, I don't mean that anyone should deliberately exclude accessibility considerations as a way to narrow their 'intended target audience'.
Once you understand your audience, you might then take Anne Gibson's alphabet of accessibility issues as starting framework for inclusive design. Would you not be able to say that your website is usable unless you cater for all of these impairments within your target audience? Wouldn't usable still seem like an impossible challenge?
Despite quotes and references that sound like it, usable is not a binary state. There is not a point at which a web site, previously considered unusable, suddenly becomes usable. Not even for a single individual user.
It's a question of the degree of usability.
If you take the quote back to source - here's the context (from Steve Krug):
People sometimes ask me, What about accessibility? Isn’t that part of
usability? And they’re right, of course. Unless you’re going to make a
blanket decision that people with disabilities aren’t part of your
audience, you really can’t say your site is usable unless it’s
You can't take that sentence as literally as Justin appeared to quote it. I'm pretty sure Krug gets fed up with people taking stuff he says quite so literally, and I've seen him say as much about various quotes in his book.
But let's even say you did exclude people with disabilities. I mean all disabilities. Blind, colour blind, short sighted, older, tired, sleep deprived, epileptic, dyslexic, short attention span and so on. Who would you be left with?
Actually, estimates are that 18% of people are impaired in some way and about 33% are temporarily impaired at any one time, and as the linked BBC GEL page says, "some days that could include you or me". In other words, today's impaired audience is not the same as tomorrow.
Still, it's about the degree of usability.
Here's some tips from the BBC Gel accessibility page:
People are different. So are impairments.
- Aim for no impassable barriers.
- Use familiarity of elements and design patterns.
- Give control over content.
- Offer choice of interaction.
- Add value for disabled users and you can help everyone.
They also offer key considerations that "will help you be inclusive of a high percentage of users most of the time".
The more you strive towards these aims, the more usable your web site will be, but there is no such thing as the binary option 'usable', and nor will you ever reach '100% usable'. We just aim do the best we can, for as many of our users as possible.