Using text-shadow for accessibility

Question

I'm interested in understanding whether there has been any research or confirmed values for using the text-shadow property with the goal of meeting WCAG minimum colour contrast values.

In my particular use case, I have a button component which doesn't meet WCAG 2.1 AA for some Whitelabel clients. I'm seeking a robust solution using text-shadow to accommodate non-conforming colour pairings.

Specifically, I'd like to know things like:

• How much shadow is enough to constitute the adjacent element in WCAG terms
• If I pick a shadow colour that adequately contrasts as a solid background, but only applies it as a shadow, is that still enough contrast? or would I need to pick a colour that contrasts in a graduated state? (maybe that's higher contrast than solid) - if that's the case, how can I calculate this?
• Are there any tools that either assess or generate confirming text-shadow values?

EDIT In answer to the third question there does appear to be a plugin for chrome which performs a pixel by pixel scan of a webpage and whilst it requires a bit of tweaking and judgement, it's about the best thing I've found for this problem: https://chrome.google.com/webstore/detail/color-contrast-analyzer/dagdlcijhfbmgkjokkjicnnfimlebcll/related?hl=en

The project is on Github here: https://github.com/gdkraus/color-contrast-chrome

Answer 1

Using text-shadow in the right way ( 4.5:1 contrast ratio between text color and shadow color ) will increase your contrast to theoretically pass the WCAG.

The issue is that it might be hard to validate the contrast with the tools.

This is what W3 says about contrast and shadow, strokes :

If the background or the letters vary in relative luminance (or are patterned) then the background around the letters can be chosen or shaded so that the letters maintain a 4.5:1 contrast ratio with the background behind them even if they do not have that contrast ratio with the entire background.

For example, if a letter is lighter at the top than it is a the bottom, it may be difficult to maintain the contrast ratio between the letter and the background over the full letter. In this case, the designer might darken the background behind the letter, or add a thin black outline (at least one pixel wide) around the letter in order to keep the contrast ratio between the letter and the background above 4.5:1.

https://www.w3.org/TR/2014/NOTE-WCAG20-TECHS-20140916/G18

As mentioned in the comment below. One way to determine the contrast would be to take the lightest area of the shadow :

r

Pick it up as a resulting color:

And run it in the contrast checker :