Enqueued Assets audit: treat non-cacheable and zero-size responses as errors #2417

Summary

The Site Health "Blocking assets" check (Enqueued Assets audit) only treats failed HTTP requests (e.g. non-200 status) as errors. It does not treat non-cacheable responses or zero-byte responses as problems, so the audit can report an asset as OK even when it is bad for performance or effectively broken.

Current behavior

In perflab_aea_get_asset_size(), the code:

1. Returns WP_Error for wp_remote_get() failures.
2. Returns WP_Error when the response code is not 200.
3. Otherwise returns strlen( wp_remote_retrieve_body( $response ) ) as the size.

So if an asset returns 200 with:

• Cache-Control: no-store (or similar non-cacheable headers), or
• Body length 0

it is still reported as successful, with no warning or error.

Proposed behavior

1. Non-cacheable response: If the response indicates the asset should not be cached (e.g. Cache-Control: no-store, no-cache, or missing cache headers where caching would be expected), treat it as an error or warning so the audit can surface it as a performance concern.
2. Zero-size response: If the response body length is 0, treat it as an error (e.g. return WP_Error or ensure the audit shows a failure) so the check does not report the asset as OK.

Code location

• File: plugins/performance-lab/includes/site-health/audit-enqueued-assets/helper.php
• Function: perflab_aea_get_asset_size() (around lines 497–528)
• Existing TODOs: Lines 525–526 already note these cases:
  • "A non-cacheable response should also be considered an error."
  • "A size of zero could be considered an error too."

Steps to reproduce (optional)

1. Install and activate Performance Lab.
2. Go to Tools → Site Health → Status and run the Blocking assets check.
3. If any enqueued script or style is served with non-cacheable headers or returns an empty body (e.g. 200 with 0 bytes), the audit currently does not flag it; it would only flag non-200 or request failures.

Acceptance criteria (suggested)

• Zero-byte response body is treated as an error (audit shows failure for that asset).
• Non-cacheable response (e.g. Cache-Control: no-store / no-cache or other agreed definition) is treated as an error or warning in the audit.
• Existing tests pass; new tests cover zero-size and non-cacheable cases where appropriate.

I’d like to work on this after it’s confirmed by the team.