Feature Policy for freezing iframes.

An explainer to define ability to freeze an iframe.

Motivation

Suppose a page contains an iframe. It is possible that the embedding document does not wish the iframe to consume CPU or network resources while not actively displayed. Controlling the behavior between two frames requires active communication and co-ordination between the frames.

Co-ordination between frames can be difficult because the only way to communicate between frames is via postMessage. So the each side has to co-ordiante on an API that allows each side to indicate when they should restrict their CPU usage.

Abstract

To improve this situation we extend the Page Lifecycle such that individual frames can be frozen.

There are two situations in which frames can be frozen:

• Frame is scrolled out of viewport
• Frame is not rendered (ie: display: none)

Since frames can begin out of the viewport and it is desirable that they be ready when scrolled into view frames will not be frozen until the load event is dispatched to the revelant frame's document.

When a frame is frozen all media (webaudio, video, audio) will be paused and begin playing again when a frame is resumed. These policies can be controlled by the feature policy values.

Doing this based on a feature policy removes the requirement for embedder to do complicated work. ie. The policy can be set and the browser will control the freezing behaviour based on those policies.

Frames when frozen will be dispatched the freeze and resume events. If documens wish to control pausing the media themselves it is suggested that they do that in their freeze and resume events.

Same origin or different origin iframes should be treated the same according to this policy. Same origin documents could present problems being directly scriptable from the parent frame. However, since the policy is explicitly opt-in it is a negotiation between a embedder and embeddee.

Feature Policy

Introduce two feature policy values:

• execution-while-out-of-viewport indicates freezing frames that aren't in the viewport
• execution-while-not-rendered indicates freezing frames that aren't rendered

Media that was paused will automatically be resumed for execution-while-out-of-viewport frames that become visible in the viewport.

Media that was paused will remain paused for execution-while-not-rendered frames when they become rendered.

<iframe allow="execution-while-out-of-viewport 'none'">
<iframe allow="execution-while-not-rendered 'none'">