Experimental: This is an experimental technology
Check the Browser compatibility table carefully before using this in production.
Non-standard: This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.
The HTTP Feature-Policy header sync-xhr directive controls whether the current document is allowed to make synchronous XMLHttpRequest requests.