There is a legal requirement to support disabilities. E.g.
Some sites make items such as manifestos available in audiobook etc. format. I'm not sure we need to go there, but we do need to be aware of accessibility considerations (will they be using a text-only browser?) and design accordingly.
Many phone systems allow for the option to invert colours etc., and most browsers allow style settings to be overwritten at the users request.
Consideration of accessibility issues also applies to menu design - does it only work with a mouse and clicking on items?
What we intend to support, and how we intend supporting it should be documented, and producible if challenged. And remember that in many cases, the user's browser settings may override what we've chosen to display.