[css-cascade-5] Draft revert-layer keyword behavior (w3c#5793)

Author: mirisuzanne

css-cascade-5/Overview.bs

@@ -1464,7 +1464,7 @@ Erasing All Declarations: the ''unset'' keyword</h4>
 	(or all longhands of a <a>shorthand</a>).
 
 <h4 id="default">
-Rolling Back The Cascade: the ''revert'' keyword</h4>
+Rolling Back Cascade Origins: the ''revert'' keyword</h4>
 
 	If the <a>cascaded value</a> of a property is the <dfn for=all caniuse="css-revert-value">''revert''</dfn> keyword,
 	the behavior depends on the [=cascade origin=] to which the declaration belongs:
@@ -1490,6 +1490,19 @@ Rolling Back The Cascade: the ''revert'' keyword</h4>
 			For the purpose of ''revert'', this origin includes the Animation <a>origin</a>.
 	</dl>
 
+<h4 id="default">
+Rolling Back Cascade Layers: the ''revert-layer'' keyword</h4>
+
+	If the <a>cascaded value</a> of a property is the <dfn for=all>''revert-layer''</dfn> keyword,
+	the <a>cascaded value</a> is rolled back to the layer below,
+	so that the <a>specified value</a> is calculated
+	as if no rules were specified in the current layer
+	for this property on this element.
+	If there are no lower-priority layers in the same [=cascade origin=]
+	as the ''revert-layer'' value,
+	the <a>cascaded value</a> may be rolled back to a previous origin,
+	as defined by the [=revert=] keyword.
+
 
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