diff --git a/css-containment/Overview.bs b/css-containment/Overview.bs
index fab7b0717546..c408961dd694 100644
--- a/css-containment/Overview.bs
+++ b/css-containment/Overview.bs
@@ -138,6 +138,37 @@ Layout Containment
2. The element must be a formatting context.
+ 3. If a fragmentation context participates in layout containment,
+ the first element with layout containment
+ affecting the fragmentation context
+ must “trap” the remainder of the fragmented flow.
+ Fragmentation must not continue past the layout containment boundary,
+ and the last fragmentation container
+ within the first layout containment boundary
+ is treated as if it is the last fragmentation container
+ in its fragmentation context.
+
+ If subsequent fragmentation containers in the fragmentation context
+ are only generated when more content remains in the fragmented flow,
+ then they are not generated.
+ If they would exist regardless,
+ they remain part of the fragmentation context,
+ but do not receive any content from the fragmented flow.
+
+ Specifically:
+ - CSS Regions following the one which traps the content
+ are still considered part of the region chain
+ as returned by the {{NamedFlow/getRegions()}} method
+ of the {{NamedFlow}} interface.
+ - the {{Region/regionOverset}} attribute of the {{Region}} interface
+ of the region which traps the content
+ is set to ''overset'' if the content doesn't fit,
+ even if it is not the last region in the region chain.
+ - If the computed value of the 'continue' property
+ on an element with layout containment
+ would otherwise have been ''continue/auto'' or ''continue/fragments'',
+ it must instead compute to ''continue/overflow''.
+
Possible optimizations that can be enabled by layout containment include (but are not limited to):
1. When the style or contents of a descendant of the containing element is changed,