IE uses named System Colors as defaults for each of the scrollbar color properties (as noted in https://www.w3.org/Style/Examples/007/scrollbars.en.html) , equivalent to:

html { 
    scrollbar-face-color: ThreeDFace;
    scrollbar-shadow-color: ThreeDDarkShadow;
    scrollbar-highlight-color: ThreeDHighlight;
    scrollbar-3dlight-color: ThreeDLightShadow;
    scrollbar-darkshadow-color: ThreeDDarkShadow;
    scrollbar-track-color: Scrollbar;
    scrollbar-arrow-color: ButtonText;
}

Should we use those literal value keywords as initial values for each property respectively instead of normal, or specify and accept both?

Minimal compat is to only specify and use the IE implemented initial values, but they're ugly (CamelCase instead of CSS shish-case), not obvious (though 'initial' keyword is a workaround for having to remember them), and hard to remember (slightly different for each property).

Currently the spec uses 'normal' instead (more like usual CSS, more obvious, easier to remember), but this is not implemented by anyone.