assertHead example

// Test ['#attached']['feed'].     $this->drupalGet('/render_attached_test/feed');
    $this->assertSession()->responseHeaderEquals('X-Drupal-Cache', 'MISS');
    $this->assertFeed();
    // Repeat for the cache.     $this->drupalGet('/render_attached_test/feed');
    $this->assertSession()->responseHeaderEquals('X-Drupal-Cache', 'HIT');

    // Test ['#attached']['html_head'].     $this->drupalGet('/render_attached_test/head');
    $this->assertSession()->responseHeaderEquals('X-Drupal-Cache', 'MISS');
    $this->assertHead();
    // Repeat for the cache.     $this->drupalGet('/render_attached_test/head');
    $this->assertSession()->responseHeaderEquals('X-Drupal-Cache', 'HIT');

    // Test ['#attached']['html_head_link'] when outputted as HTTP header.     $this->drupalGet('/render_attached_test/html_header_link');
    $expected_link_headers = [
      '</foo?bar=<baz>&baz=false>; rel="alternate"',
      '</foo/bar>; hreflang="nl"; rel="alternate"',
      '</foo/bar>; hreflang="de"; rel="alternate"',
    ];