This feature is experimental. Please share your experiences so we know what to keep or change.
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.