#Contrastou is a free Chrome add-on developed by Kelly Gonzaga. It falls under the category of Browsers and is specifically categorized as an Add-ons & Tools. This add-on allows users to measure the contrast ratio between two colors and determine if the combination is accessible according to the WCAG guidelines.

With #Contrastou, users can simply input the hexadecimal values of the background color and the text color, or use the color picker tool to select the colors directly. The add-on then calculates the contrast ratio between the two colors. In order for the combination to be considered accessible, there are minimum values that need to be met. These values are lower if the font size is larger than 24px or larger than 19px with bold.

Additionally, #Contrastou provides a visual representation of the contrast ratio right within the extension. This allows users to quickly and easily see the result of their color configurations.