The sync-script policy controls whether a document will execute parser-blocking JavaScript. This includes both external <script src=''> resources that do not have the async or defer attributes, but also any inline script that is not an ES module, and which does not have a defer attribute. Blocked scripts will be skipped by the parser completely, ie. they are not simply forced to execute asynchronously, since that may result in unexpected behaviour.

How to apply this policy

Send the following HTTP header to control the sync-script policy, and disallow it on all origins:

Feature-Policy: sync-script 'none'

Does it work?

Currently Firefox, and Chromium based browsers, such as Google Chrome, Samsung Internet, and Opera, are the only user-agents to support Feature Policy. The minimum version that correctly recognises the sync-script policy is:

Mozilla Firefox


Google Chrome


Microsoft Edge

Not supported

Apple Safari

Not supported