WebAIM - Web Accessibility In Mind

E-mail List Archives

Colour contrast measurement techniques

for

From: Steve Green
Date: Feb 17, 2023 3:23PM

The Understanding page for WCAG SC 1.4.3 contains a note that says "Because authors do not have control over user settings as to how text is rendered (for example font smoothing or anti-aliasing), the contrast ratio for text CAN be evaluated with anti-aliasing turned off."

It says CAN rather than MUST, so there is no "right" way to make the measurement. For many years I used to measure the rendered colour, but I now record the author specified colour. To a large extent that is because anti-aliasing varies across browsers and operating systems and also because the colour you pick is highly arbitrary. If you pick one colour and someone else (usually the developer) picks another colour that gives a different pass/fail result, who is right?

I would be interested to hear how everyone makes the measurement to see if there is any sort of consensus. Do you use the author specified colour? Or pick a colour that approximates to how you perceive the overall colour to be? Or zoom the text so it is no longer anti-aliased? Or something else? What if none of the pixels approximate to how you perceive the overall colour to be?

There are some unusual cases where you probably have to measure the rendered colour, such as when text is viewed through a translucent layer, but I am mostly interested in the ordinary measurements we make every day.

Steve Green
Managing Director
Test Partners Ltd