[css-anchor-position-1] Treat auto insets as zero for finding IMCB size for position-try-options #12942

Author: fantasai

css-anchor-position-1/Overview.bs

@@ -1665,7 +1665,8 @@ rather than strictly following the order declared in 'position-try-fallbacks'.
 		For each entry in the [=position options list=],
 		[=apply a position option|apply that position option=] to the box,
 		and find the [=inset-modified containing block=] size
-		that results from those styles.
+		that results from those styles
+		(treating ''inset/auto'' 'inset' values as zero).
 		Stably sort the [=position options list=]
 		according to this size,
 		with the largest coming first.