This request header contains the target selector for a successful fragment update.
Server-side code is free to optimize its response by only rendering HTML
that matches the selector. For example, you might prefer to not render an
expensive sidebar if the sidebar is not targeted.
Unpoly will usually update a different selector in case the request fails.
This selector is sent as a second header, X-Up-Fail-Target.
Example
The user updates a fragment .menu.
Unpoly automatically includes the following request header:
X-Up-Target: .menu
The server chooses to render only the HTML for the updating fragment.
It responds with the following HTTP:
Vary: X-Up-Target
<div class="menu">...</div>
Note
Request headers that influenced a response should be listed in a Vary response header.
This tells Unpoly to partition its cache for that URL so that each
request header value gets a separate cache entries.
Changing the render target from the server
The server may change the render target context by including a CSS selector as an X-Up-Target header
in its response.
Content-Type: text/html
X-Up-Target: .selector-from-server
<div class="selector-from-server">
...
</div>
The frontend will use the server-provided target for both successful (HTTP status 200 OK)
and failed (status 4xx or 5xx) responses.
Rendering nothing
The server may send an X-Up-Target: :none response header with an empty body to skip the current render pass.
Also see skipping unnecessary rendering.
