[chipotle_coyote]

I use the WebAIM contrast checker:

https://webaim.org/resources/contrastchecker/

It's basically a measure of perceived brightness. The actual formula involves calculating the relative luminance of the background and foreground colors, where luminance is a value from 0 (darkest) to 1 (lightest), and using the formula

(lighter_luminance + 0.05) / (darker_luminance + 0.05)

So the highest possible contrast ratio in this system is 21:1.

[dotancohen]

I see, thank you.