[css-color][css-ui] Better Fallback for outline-color: invert

[Crissov]

The support for outline-color: invert is very limited #423, because, as I understand it, it is complicated to achieve the effect with current technology while it used to be simple when fewer layers of abstraction stood between hardware and browser. CSS-UI therefore allows browsers to ignore the value altogether. Can we perhaps do a little better? I propose that browsers should, instead, fall back, to inverting the computed value of the background-color property. In some simple cases with the same plain background, this will actually yield the expected visual result.

With this convention in place, invert might even become useful as a keyword for other *-color properties, but, to avoid cycles, not for background-color itself and thus not for the <color> type.

• caret-color
• color
• text-emphasis-color
• text-decoration-color
• border-color …
• column-rule-color
• stroke-color
• fill-color

Plans and proposals for more sophisticated color modifications should not be affected by this.

[Crissov]

This might create an infinite loop then:

foo {
  color: invert;
  background-color: currentcolor;
}

PS: CSS UI implies external knowledge about exactly what it means to invert a color, probably rgb(calc(100% - red(currentcolor)) calc(100% - green(currentcolor)) calc(100% - blue(currentcolor)) / opacity(currentcolor)).

[frivoal]

Most of the time, the background color is transparent. What do you do then?

[Crissov]

Since transparent is defined as transparent black, the inverse would probably be transparent white, which would not be very useful indeed. In many cases, it would perhaps be useful to fall back to currentcolor instead.