Rec.2020 is a color primaries standard from the ITU, defined as part of the UHDTV specification. Its red, green, and blue primaries sit much further out on the visible-color horseshoe than sRGB's or even Display P3's — theoretically covering around 75% of all colors the human eye can perceive, versus roughly 35% for sRGB.
Rec.2020 vs. Rec.2100 — a common mix-up
Rec.2020 defines color primaries only — where red, green, and blue sit on the color chart. It says nothing about brightness or transfer functions. Rec.2100 is the fuller HDR system spec that pairs those same Rec.2020 primaries with PQ or HLG for actual HDR video and images. You'll often see the two conflated, but they answer different questions: Rec.2020 is "which colors," Rec.2100 adds "how bright."
How this shows up in our tools
Our Inspect tool checks for Rec.2020 primaries the same honest way it checks for Display P3 — real CICP metadata for AVIF, real ICC profile primaries for JPEG/PNG where present, and an explicit "unconfirmed" rather than a guess when neither exists. We don't currently render Rec.2020 content at its real gamut anywhere in the pipeline — browser Canvas 2D doesn't expose a Rec.2020 color space at all, only sRGB and Display P3 — and we say so rather than pretending otherwise.