[css-grid-2] Clarify how relpos applies to subgrids. w3c#7123

Author: fantasai

css-grid-2/Overview.bs

@@ -3903,6 +3903,12 @@ Subgrids</h2>
 			The 'overflow' property does apply to <a>subgrids</a>,
 			so that overflowing contents of the subgrid can be scrolled into view.
 			(Note: the act of scrolling does not affect layout.)
+
+		<li id="subgrid-relpos">
+			[=Relative positioning=] applies to <a>subgrids</a> as normal,
+			and shifts the box and its content together as usual.
+			(Note: Relative positioning takes place after alignment,
+			and does not affect track sizing.)
 	</ul>
 
 <!--
@@ -5393,6 +5399,18 @@ Changes</h2>
 Changes since the <a href="https://www.w3.org/TR/2020/CRD-css-grid-2-20201218/">18 December 2020 CR</a></h3>
 
 	<ul class="non-normative">
+		Clarified how [=relative positioning=] applies to [=subgrids=].
+		(<a href="https://github.com/w3c/csswg-drafts/issues/7123">Issue 7123</a>)
+		<blockquote>
+			<ul>
+				<li>
+					<ins>[=Relative positioning=] applies to <a>subgrids</a> as normal,
+					and shifts the box and its content together as usual.
+					(Note: Relative positioning takes place after alignment,
+					and does not affect track sizing.)</ins>
+			</ul>
+		</blockquote>
+
 		<li id="change-2020-flex-intrinsic-ratios">
 			Changed intrinsic track space distribution <a href="#algo-spanning-flex-items">across flexible tracks</a>
 			for flex sums between zero and one to interpolate