(Aside: “96.61%” is… well, for the type of browser they’re talking about—ones that support such things as CSS and colour—it should be exactly 100%, if not higher. Decide for yourself the sanity of this usage figure.)
CSS Color Module Level 4 <https://www.w3.org/TR/css-color-4/#css-system-colors> revived the concept of system colours, largely fixing the bad aspects and making it all clearly good. Some of the colours are still deprecated, with some at-risk³ dumbing-down. But there are also some new colours: Canvas and CanvasText.
So the problem is that that MDN compatibility data is talking about CSS2 system colours, not CSS Color Module Level 4, which has much lower support (and may not all have been implemented at the same time: most were probably added around 2019, but some in 2021 and AccentColor and AccentColorText in 2022).
Here are the actual colours, to help anyone who wants to update data (have fun identifying when they were added!):
• New in CSS Color Module Level 4: AccentColor, AccentColorText, ActiveText, ButtonBorder, Canvas, CanvasText, Field, FieldText, LinkText, Mark, MarkText, SelectedItem, SelectedItemText, VisitedText.
• Retained from CSS2: ButtonFace, ButtonText, GrayText, Highlight, HighlightText.
¹ Look, some of the system colours were clearly bad, modelling OS styles that were almost completely gone by this time. I suspect that some people didn’t like some of these too-specific values, and they threw the baby out with the bathwater.
² Earlier revisions mentioned the deprecation as being in favour of the ‘appearance’ property, but this was pretty stupid⁴ even when it was prospective, as first written, long before the ‘appearance’ property bit the dust for that time.
³ See <https://www.w3.org/TR/css-color-4/#deprecated-system-colors>: it defines fallback mappings that reduce the variations, but Firefox and Safari haven’t implemented the dumbing-down, still having the more-nuanced stuff they had from decades go. Hence “Results on these 'same as' equivalence tests are not great, which is why the feature is at-risk”.
⁴ No, seriously, just totally clueless and I have no idea what whoever wrote that was thinking, because it was always a completely different thing with only minimal potential overlap in uses.
The article points out that while system colors are supported in Firefox (as of the time of the writing of the article), the "color-scheme: light dark" method he was using was not supported, and it looks like that was not supported until Firefox 96 (January 2022).[0]
You could use the shorthand, but it's really not necessary at this point.
The spec is (unsurprisingly) quite [specific][1] that this color value is exactly "#663399". All browsers which implement CSS Colors have had support for years now and the Color Module itself is now a CR as of a year and a half ago.
Incompatibility isn't exactly a rounding error yet, but we're close.
I've used something to this effect before, and you can use child selectors on e.g images to effectively exclude elements.
Note that the cost of this lazy method is very poor performance (css filter over the entire document), and lack of control over dark colours (inverted colours don't necessarily work best, shifting hue can help but you probably don't want the same as uninverted colours because they tend to be too dark and lack sufficient contrast).
If you use just invert(100%), no grayscale, on the body, you can add img { filter: invert(100%) } to reverse images back to their original color. However, if an image has any transparency or translucency, it may be harder to see with what the background color has become.
https://caniuse.com/mdn-css_types_color_system-color