[css-anchor-1] Define how top layer and anchors interact.

Author: tabatkins

css-anchor-1/Overview.bs

@@ -20,6 +20,24 @@ spec:css-position-3;
 	type:property; text:inset-inline-start
 </pre>
 
+<style>
+/* Put nice boxes around each algorithm. */
+[data-algorithm]:not(.heading) {
+  padding: .5em;
+  border: thin solid #ddd; border-radius: .5em;
+  margin: .5em calc(-0.5em - 1px);
+}
+[data-algorithm]:not(.heading) > :first-child {
+  margin-top: 0;
+}
+[data-algorithm]:not(.heading) > :last-child {
+  margin-bottom: 0;
+}
+[data-algorithm] [data-algorithm] {
+    margin: 1em 0;
+}
+</style>
+
 Introduction {#intro}
 =====================
 
@@ -442,21 +460,40 @@ might not be capable of anchoring the positioned element.
 	but the details of that will depend on the API assigning them.)
 </div>
 
-<div algorithm>
+<div algorithm="acceptable anchor element">
 	An element |el| is a <dfn export>acceptable anchor element</dfn>
 	for an [=absolutely positioned=] element |query el|
-	if all of the following properties apply:
+	if any of the following are true:
+
+	* |query el| is in a higher [=root layer=] than |el|.
+
+	* |query el| and |el| are in the same [=root layer=],
+		and all of the following are true:
+
+		* Either |el| is a descendant of |query el|'s [=containing block=],
+			or |query el|'s [=containing block=] is the [=initial containing block=].
+
+		* If |el| has the same [=containing block=] as |query el|,
+			|el| is not [=absolutely positioned=].
 
-	* Either |el| is a descendant of |query el|'s [=containing block=],
-		or |query el|'s [=containing block=] is the [=initial containing block=].
+		* If |el| has a different [=containing block=] from |query el|,
+			the last [=containing block=] in |el|'s [=containing block chain=]
+			before reaching |query el|'s [=containing block=]
+			is not [=absolutely positioned=].
 
-	* If |el| has the same [=containing block=] as |query el|,
-		|el| is not [=absolutely positioned=].
+	For the purposes of this algorithm,
+	an element is in a particular <dfn noexport>root layer</dfn>
+	corresponding to the closest [=inclusive ancestor=]
+	that is in the [=top layer=],
+	or the document if there isn't one.
+	[=Root layers=] are "higher" if their corresponding element
+	is later in the [=top layer=] list;
+	the layer corresponding to the document is lower than all other layers.
 
-	* If |el| has a different [=containing block=] from |query el|,
-		the last [=containing block=] in |el|'s [=containing block chain=]
-		before reaching |query el|'s [=containing block=]
-		is not [=absolutely positioned=].
+	Note: This wording around "root layer"
+	needs to live in a stacking-context spec,
+	after pulling the [=top layer=] stuff
+	out of [[fullscreen]].
 </div>
 
 An element can also have an <dfn export>implicit anchor element</dfn>,