It wasn't obvious to me -- I misread "blue" as "white".

`#fff` is device color, it's short for `#ffffff` which is 24-bit RGB that predates sRGB, as does true color device support. I was sending 24-bit RGB to VESA-compliant graphics cards before sRGB became a thing. `#fff` was supported by Photoshop and Macromedia products as straightforward device colour format, before sRGB was adopted by at least the latter, mind you. The use by CSS is co-incidental, not where the format was introduced.