There is a great video from BSI about the importance of accessibility in website design:
A couple of points made in the video which I find particularly interesting are: most (probably all) website designers understand the importance of accessibility but many (most?) of them really do not realise what it entails and how to ensure they are doing it; and many people who would not otherwise classify themselves as disabled find themselves disadvantaged and excluded by poor design choices and lack of site testing – with developers failing to test their sites to see whether they are indeed accessible.
This is becoming a bit of a cause of mine lately as I have congenital nystagmus (which in my case does not often cause me difficulties). However, currently on one of my projects I am trying to use an online resource which in its current iteration appears almost specifically designed to stimulate nystagmus (involuntary wandering/darting eyes) making it pretty much inaccessible to me.