This feature is experimental. It may be changed or removed in a future version.
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.
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.
See Conditional requests for a full example.
The fragment's last modification time.
Return undefined
if the fragment was rendered without a modification time.