Returns the last modification time of the content in the given element.
The last modification time corresponds to the Last-Modified header in the response that
rendered the fragment. Alternatively the [up-time] attribute of the element or an ancestor is used.
If no ancestor fragment with an [up-time] attribute exists, undefined is returned.
When the fragment is reloaded,
its modification time is sent as an If-Modified-Since request header. The server may check the header and decide to skip rendering.
Conditional requests
