The Web Accessibility Initiative (WAI) created the Web Content Accessiblity Guideline (WCAG) to provide a standard to meet the accessibility needs if individuals, organizations, and government. The linked document explains how to make content more accessibile, but that's a lot of pages so I am going to walk through my choices.
A 'good' font must have easily distinguishable characters , a tall x height, a height difference between numerals and capital letters, and adequate letter spacing. A study by Thomas Bohm found that people aged 13 to 45 without dyslexia had trouble differentiating between the following letters:
The font used on this page meets all of those criteria, with easily distinguished characters, as seen above, a tall x-height, a height difference between numerals and capital letters, as seen between "I" and "1", and adequate letter spacing, as seen in the text.
Source: What makes a good, accessible, easy to read font? (GatherContent.com)
An accessible website will have a contrast ratio of at least 4.5:1, ensuring that colorblind viewers can read the text. On this page, the white background and black text has a ratio of 21.0:1 while the Sewanee Purple background with white letters has a ratio of 11.528:1, both of which exceed the minimum 4.5:1 ratio. Tangu Contrast-Finder is a great resource to check the color contrast between different colors.
Source: Contrast & Color (web-accessability.carnegiemuseums.org)