Re: [w3c/manifest] Rename `purpose: badge` to `purpose: monochrome` and expand on it (#830)

Hi Tiger,

Thanks for opening this discussion. I am pretty sure you meant rename to "monochrome", not "maskable" (???), so I've gone ahead and renamed the issue. :) This is a bit of a dupe of #795 but since that discussion is very long and shortcut-focused, happy to start a fresh one here. However, readers should be aware that this was litigated a fair bit on #795.

I completely agree with the proposal here. Happy to work on spec language for it (you can do it and I can review it, or I can do it with your review if you're happy for me to take it). We need to specify exactly the format of a monochrome icon so that UAs can adapt it for their various purposes, e.g. Safari wants to make a grey gradient, Windows might want to change the icon's foreground color to the app's theme_color, etc, etc. But in order to do that, we need to establish a contract between the developer and the UA, so that for example it's well defined what the foreground pixels and the background pixels are.

I'm happier to go with your suggestion of ignoring RGB pixel colour and just looking at the alpha channel (over my previous suggestion on #795 to ignore alpha and ask for icons to be greyscale). It does make the icons a little harder to author, but it's much less ambiguous (for example, we don't need to worry about what happens if we find a non-grey pixel; we just totally ignore RGB channels and only look at alpha).

So the specifics would be that the UA MUST NOT look at the RGB channels of the pixel data (developers are recommended to set all pixels to black, but it doesn't matter). It SHOULD treat the alpha channel with opaque = foreground colour, and transparent = background colour. UAs are free to manipulate the image however they see fit, for example, converting the transparency into a solid colour, or setting all of the RGB pixel data to white, or to the app's theme colour.

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