[css-position-3] Abspos unsafe-aligns in IMCB, safe-aligns in CB. w3c#10316

Author: fantasai

css-position-3/Overview.bs

@@ -882,7 +882,8 @@ Absolute Positioning Layout Model</h2>
 			First, its [=inset-modified containing block=] is calculated,
 			defining its [=available space=].
 			(See [[#abspos-insets]].)
-			Because an [=absolutely positioned box=] can have no effect on the size of its [=containing block=],
+
+			Because an [=absolutely positioned box=] does not affect the size of its [=containing block=],
 			its [=available space=] is always [=definite=].
 		<li>
 			Next, its width and height are resolved against this [=definite=] [=available space=],
@@ -898,6 +899,13 @@ Absolute Positioning Layout Model</h2>
 		<li>
 			Lastly, its [=margin box=] is aligned within the [=inset-modified containing block=]
 			as defined by its [=self-alignment properties=].
+
+			For this purpose, if neither ''<overflow-position>/safe'' nor ''<overflow-position>/unsafe'' is specified,
+			the box aligns at an "in-between" level of safety:
+			it is ''unsafe''-aligned within its [=inset-modified containing block=],
+			but shifted, if necessary, to not overflow
+			the union of its [=absolute-position containing block=] and its [=inset-modified containing block=]--
+			unless it is larger than that rectangle, in which case it is start-aligned to it.
 	</ol>
 
 <h3 id="abspos-auto-size">
@@ -1691,6 +1699,10 @@ Acknowledgments</h2>
 	The following significant changes were made since the
 	<a href="https://www.w3.org/TR/2023/WD-css-position-3-20230403/">3 April 2023 Working Draft</a>:
 	<ul>
+		<li>Adjusted alignment of [=absolutely positioned boxes=]
+			to, by default, constrain the box to its [=containing block=].
+			See [[#abspos-layout]].
+			(<a href="https://github.com/w3c/csswg-drafts/issues/10316">Issue 10316</a>)
 		<li>Clarified that the [=fixed positioning containing block=]
 			corresponds to the [=layout viewport=] (specifically) in [=continuous media=].
 			(<a href="https://github.com/w3c/csswg-drafts/issues/6453">Issue 6453</a>)