On https://css-tricks.com/new-css-color-features-preview/, a commenter asks:
"...why we need display-p3, and why we canโt express all colors with RGB anymore?"
My response:
We were never able express all colors with RGB.
RGB in the browser was using sRGB, which is a certain mapping of R,G,B numbers to a subset of all colors.
Display P3 is a different mapping of R,G,B numbers to a different subset of all colors.
A given display monitor can only display a certain subset of all colors.
sRGB used to be sufficient, but newer monitors support a larger subset of all colors, so Display P3 is needed. Just re-mapping sRGB to a wider range would break all previous colors that developers put in their apps.