This feature is experimental. It may be changed or removed in a future version.
Returns the ETag of the content in the given element.
The ETag corresponds to the ETag header
in the response that rendered the fragment. Alternatively the [up-etag] attribute of the element
or an ancestor is used.
When the fragment is reloaded,
its ETag is sent as an If-None-Match request header. The server may check the header and decide to skip rendering.
See Conditional requests for a full example.
The fragment's ETag.
Return undefined if the fragment was rendered without an ETag.