Help:Contrast/Advanced

Understanding visual acuity
To make your wiki easier for people to read, you need to know a bit about visual acuity — a measurement of how well you can read black letters on a white background from a particular distance.

Think about your eye doctor's office. In the US, patients are seated 20 feet from the chart. Visual acuity is a fraction where the numerator is 20 and the denominator is the number assigned to that line on the chart, Statistically average visual acuity is being able to read the line marked 20 from 20 feet away: 20/20 — or 6/6, if you use the metric system.

There's math in them thar hills
The visual acuity fraction is directly related to what's known as a contrast ratio — a mathematical expression of the comfort a person has in reading your wiki.

Put another way, a contrast ratio is the relationship between the brightnesses of the background color and the text color. The highest ratio is 21:1, which is black text on a white background, while the lowest is 1:1 — where both background and text are the same color. A person with 20/20 vision could probably see a page with a 2:1 contrast ratio, but it wouldn't be pleasant. So the minimum contrast ratio for the normally-sighted is 3:1.

But you can't deliver a wiki that's only good for people with 20/20 vision. That's why the World Wide Web Consortium (W3C) recommends two standards: AA and AAA.

With AA, the aim is to account for people who have 20/40 vision by providing a CR of 4.5:1 for regular text, and 3:1 for larger copy. AAA has a minimum contrast ratio of 7:1. This higher rating will allow people with 20/80 vision to comfortably use your wiki. Those with worse than 20/80 vision usually employ additional assistive technologies, making that the lowest visual acuity that needs to be considered by your CSS alone.