What should we do when revert-layer is used in the style attribute?

For example, <div style="color: red; color: revert-layer">. I can think of two options:

1. Treat style attribute as if in its own layer (though importance is handled differently), so we revert color to the highest author layer
2. Style attribute is not in any layer, so color: revert-layer is an invalid declaration, and color: red wins the cascade

I prefer option 1 because it is simpler to implement (no parse-time special-casing). And other than an obscure way to cancel other declarations in the style attribute, I'm not sure if there is any real use case of putting revert-layer in the style attribute, so I prefer keeping things simple.

@mirisuzanne