accessible to all
Accessibility means accessible to people with disabilities, for example, users with vision impairment who need to set their browsers to magnify the text; those using Braille readers; and people accessing the Web with screen readers or speech browsers. The RNIB estimates that 2 million people in the UK have significant vision impairment and find many Web sites hard to use. Some people have reading difficulties, e.g. dyslexia, or motor difficulties, e.g. difficulty positioning a mouse accurately.
In the UK there are two Acts of Parliament which impact on the accessibility of Web sites: the Disability Discrimination Act 1995 (Section 3) and the Special Educational Needs and Disability Act 2001. Briefly, any organisation providing a service has a legal obligation to take all reasonable steps to ensure that the service is available to people with disabilities.
But accessibility is just a subset of usability. Everyone, not just those with a disability, views Web sites under widely varying conditions. Are they using a desktop computer, laptop, tablet or mobile? What browser are they using? Vivaldi? Firefox? Chrome? Microsoft Edge? Safari? What size monitor or screen do they have? How big is their browser window?
Some Web users have temporary disabilities, e.g. people who are on the move and who use a mobile with a slow Internet connection; or people working in low-light condtions.
Many graphically attractive sites are low in usability and are difficult to read and use, even under "normal" conditions. The majority of sites are still designed as though their pages are fixed-size sheets of paper, whereas they in fact need to be rendered using the very fluid medium that constitutes the Web. Web users have zero patience (or less) and will go elsewhere if your site gives them the slightest problem! Potential clients will assume your site is at fault, not their mobile/tablet/monitor/computer/browser.