There is usually headroom above SDR white for HDR highlights, which is what people usually want. The common scRGB colorspace, often used on windows for games,

There is usually headroom above SDR white for HDR highlights, which is what people usually want. The common scRGB colorspace, often used on windows for games, defines #FFF = 1,1,1 as 200 nits. But it uses floats instead of integers so you can go much higher. You can also go below black = #000.