 Changes to certain properties of elements trigger layout style or paint in the browser. For example, when you change the font size of an element, the browser will have to reposition all siblings and repaint them. And this can get fairly expensive really, really quickly. Containment allows you to define boundaries for the browser's layout style and paint work. You're making a guarantee to the browser that changes inside an element with a containment property do not need to be propagated to the outside. In our router example, I created a CSS animation for font size, which you should never ever do, but it is an easy way to trigger layout on every frame. If we make a timer recording, we can see that the document root gets layouted every frame. So let's give every view the contain strict CSS attribute meaning that a view is now a boundary for layout style and paint. Now the timer recording will show that the view itself is the layout root. And there you have it, a single CSS property to make your performance better. What more could you possibly want? Thanks for watching and see you next time.