[css-contain-3] Relative units in dimensional queries

Author: mirisuzanne

css-contain-3/Overview.bs

@@ -238,7 +238,7 @@ Creating Query Containers: the 'container-type' property</h2>
 
 		The ''40em'' value used in the query condition
 		is relative to the [=computed value=] of 'font-size'
-		on the ''section'' container.
+		on the relevant [=query container=].
 	</div>
 
 	<div class=example>
@@ -401,7 +401,9 @@ Container Queries: the ''@container'' rule</h2>
 		to just those with a matching [=query container name=].
 	3. Select the nearest remaining [=query container=] ancestor.
 	4. If no ancestor is an eligible [=query container=],
-		then the [=container query=] is ''false'' for that element.
+		then the [=container query=] is ''unknown'' for that element.
+
+	Issue: Do missing containers evaluate false or unknown?
 
 	If an eligible [=query container=] has been selected,
 	each [=container feature=] in the <<container-query-list>>
@@ -426,7 +428,7 @@ Container Queries: the ''@container'' rule</h2>
 	</div>
 
 	Issue(6393): Provide a syntax to query a specific container-type?
-
+	Issue: Define how nested container queries are evaluated
 
 <h2 id="container-features">
 Container Features</h2>
@@ -447,13 +449,41 @@ Dimensional Container Features</h3>
 
 	If the [=query container=] does not have a [=principal box=],
 	or the principal box is not a [=layout containment box=],
+	or the [=query container=] does not support [=dimensional queries=] on the relevant axes,
 	then the result of evaluating any dimensional container feature is ''unknown''.
 
-	[=Relative length=] units in [=container queries=]
-	are based on the the [=computed values=] of the [=query container=].
+	[=Relative length=] units in [=container query=] conditions
+	are evaluated based on the the [=computed values=] of the [=query container=].
 
 	Note: This is different from the handling of relative units in [=media queries=].
 
+	<div class=example>
+		For example, [=query containers=] with different font-sizes
+		will evaluate ''em''-based queries relative to their own font sizes:
+
+		<pre class=lang-css>
+		aside, main {
+		  container-type: inline-size;
+		}
+
+		aside { font-size: 16px; }
+		main { font-size: 24px; }
+
+		@container (width > 40em) {
+		  h2 { font-size: 1.5em; }
+		}
+		</pre>
+
+		The ''40em'' value used in the query condition
+		is relative to the [=computed value=] of 'font-size'
+		on the relevant [=query container=]:
+
+		* For any ''h2'' inside ''aside'',
+			the query condition will be true above ''640px''.
+		* For any ''h2'' inside ''main'',
+			the query condition will be true above ''960px''.
+	</div>
+
 	Issue: Evaluate to "unknown" if inline-/block-/size containment is missing.
 
 <h4 id="width">