And no, #fff is not a "device color". The syntax originates from the web where sRGB is implied ever since we had displays brighter than that.
`#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.