[webcomponents] Clarify shadow DOM algorithms (#410)

My group has found shadow DOM spec to be confusing about the order that the slotting and flattening algorithms are applied.  Is each step applied to all of the nodes before continuing to the next step?

Perhaps changing to this statement:
The slotting algorithm must be used to determine the assigned slot for **each** node and must be equivalent to processing the following steps


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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/410