There’s this thing called the WCAG: Web Content Accessibility Guidelines. They set out all the good things you need to do to make your content accessible for people with disability, people including those with vision impairment (an estimated 300,000 in Australia alone), and also people with other kinds of disability, such as cognitive or motor impairment.
I read an accessibility report about a certain website today, and was very impressed at the thoroughness of the company who conducted it. (I did have a look at the styles and layout in the report itself, and couldn’t catch them out. Good job!)
Not mentioned (and possibly not relevant to that particular report) was an often overlooked aspect of accessibility: grade level readability.
Paraphrasing the W3C:
After proper nouns (place and person names, things that there are only 1 of, etc.) are removed, text should be understandable by a person who has had 9 years of schooling. (i.e. a Grade 9 level Flesch–Kincaid readability score).
Note, this is paraphrasing, and there are some exceptions to this, but it’s a good rule to follow.