Returns the ETag of the content in the given element.
The returned 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.
If no ancestor fragment with an [up-etag] attribute exists, undefined is returned.
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.
Conditional requests
