This response header contains a hash identifying the content in the response body.
Typically, the ETag value is a hash of the underlying data that was rendered,
or a hash of the data's last modification time.
ETags can be used to prevent unnecessary re-rendering of unchanged content.
See Conditional requests for details and examples.
A (weak) ETag typically looks like this:
ETag: W/"55e4d42a148795d9f25f89d4"
For more information about an ETag's format, see
MDN and RFC 7232.
Alternatives
Instead of sending this header, the server may also render fragments with [up-etag] attributes.
