This seems a little involved because of ARC++ notifications. 

Thanks Sébastien! I'm starting to take a look at this. I'm trying to reconcile the way shadows are defined in the specs and the way they are implemented. For instance, the code defines a "shadow elevation" as a single number. Some standard values are defined for the whole window manager:

----

// Standard shadow elevations used by the the aura window manager. The value is
// used to initialize an instance of wm::Shadow and controls the offset and blur
// of the shadow style created by gfx::ShadowValue::MakeMdShadowValues().
constexpr int kShadowElevationMenuOrTooltip = 6;
constexpr int kShadowElevationInactiveWindow = 8;
constexpr int kShadowElevationActiveWindow = 24;

----

If possible I'd like to remain within what's defined as standard values, or add a new one if necessary. Do you know how this connects to numbers in the spec (which define shadows more like in CSS)?

Removing Google restriction, I don't really see any issue with this happening in the open. Do let me know if you disagree.

My assumption after working on this with estade@ is that he has built the shadow system in Chrome OS based on the specification you see in #0. Shadows are built with a combination of an Ambient + Key shadow, as defined in this spec:
https://gallery.googleplex.com/projects/MCHbtQVoQ2HCZVwwz_9lG2u2/files/MCG6rbSUEnzjjeAb6qo-x28pwMSJbxAVNpw

This means that if you use kShadowElevationActiveWindow = 24, it will render the combination of shadow which approximates the CSS equivalent, in SKIA:

Ambient: 0 0px 24px 0 rgba(0,0,0,0.12)
Key shadow: 0 24px 24px 0 rgba(0,0,0,0.24)

So what I've defined in this spec is to use "constexpr int kShadowElevationActiveWindow = 24;" which is consistent with the pre-defined standard value.

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I do not know if this system is flexible beyond the three values 6,8 and 24, meaning if it will automatically generate the shadow elevation and blur value, if I set shadow = 40 this means it should render this:

Ambient: 0 0px 40px 0 rgba(0,0,0,0.12)
Key shadow: 0 40px 40px 0 rgba(0,0,0,0.24)

Could you help me figure this out? This would help me understand how shadows work and enable me to report that in the guidelines.