What “safe” did not cover
The palette was defined in the mid-1990s, when most displays were 8-bit and could show 256 colors at once. Windows and Mac reserved different slots for their own interface chrome, so only the intersection could be relied on. Forty colors were left out of the 256 — and even among the remaining 216, a few values still failed.
Internet Explorer 6–8 on Windows 256-color mode
IE rendered these four values incorrectly, which is why Adobe and others documented only 212 truly safe colors rather than 216. If you are matching an old 8-bit reference, skip them.
Netscape Navigator on Unix (SunOS) 8-bit workstations
The Unix build shipped with a different system palette, so the Windows/Mac steps did not apply. Colors meant to survive there used five steps instead of six — 00, 40, 80, BF, FF — a 125-color set, not this 216.
Safari on wide-gamut displays macOS and iOS
Not a per-color failure. Safari maps untagged sRGB hex into Display P3, so mid-saturation colors read slightly more saturated than the same value in Chrome. It affects the whole palette; anchoring with color() is the only way to pin it down.
Firefox color management off by default
Ships with OS-level color management disabled, so gamma handling differs and mid-tones can look flatter than in Chrome. Again a whole-palette shift rather than specific values.
Chrome and Edge sRGB
Treat sRGB values faithfully. No color in this palette fails here — and on any 24-bit display, none of the 216 needs special handling at all.
Bottom line: the web-safe palette was a fix for 8-bit hardware. On today's screens every hex value renders; the constraint only matters for legacy systems, indexed images, e-ink readers and deliberately retro artwork.