> sRGB does /not/ explicitly specify what full red, green and blue are.
>
> What sRGB does define is the _xy coordinates_ - i.e. the absolute hue
> and saturation - of red, green and blue (aka the "primaries"). This can
> be visualized as defining the direction (but not the "length") of the
> red, green and blue axes in XYZ space.
>
> sRGB also /does/ explicitly define the _xy coordinates_ of the so-called
> "illuminant whitepoint" - i.e. which defines what colours are nominally
> "neutral", i.e. entirely desaturated - and also defines that such
> neutral colours are to be represented by the red, green and blue
> channels all set to the same value (which is typical for RGB colour
> models). This can be visualized as defining the direction (but again not
> the "length") of the RGB "cube"'s diagonal in XYZ space.
>
> sRGB also /does/ explicitly define the _Y coordinate_ - i.e. the
> luminance - of the brightest representable colour: 80 cd/m^2; however,
> not everyone adheres to this.

The exact definition may not be as I simplified to, but the point is the 
same. The "output" of the spec is that XYZ for each of R,G,B are numbers 
that are always the same, there is no dependence on some externally 
provided white point information.

The only additional piece of information you may want to know is if it 
has been scaled above the 80 cd/m^2 standard. eg with an "sRGB" computer 
monitor these are typically nearer 300 cd/m^2, but knowing that you can 
calculate the true colours, there is no need for any additional white point.

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