Recommend working group provide guidance on calculating contrast of rgba

[mraccess77]

Many webpages apply an alpha channel to rgb colors in order to lighten the color of text. This is prolific as light gray text is wildly popular among designers. Applying this opacity lightens the text and some contrast checking tools ignore this value.

It would be helpful to update the understanding text contrast document and techniques to remind testers and authors that the alpha channel must be taken into consideration when determining contrast.

There is a formula for converting rgba to rgb, a random example here http://marcodiiga.github.io/rgba-to-rgb-conversion

[alastc]

I think a paragraph in the understanding document, and maybe a note in the main techniques would do it. Can anyone draft those?