diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 9e6af5d3..4345a2e3 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -42,6 +42,8 @@ jobs:
 
             - name: Tests
               run: composer test
+              env:
+                  EMBED_STRICT_CACHE: 1
 
     phpstan:
         name: PHPStan Static Analysis
diff --git a/README.md b/README.md
index 538a0c46..57dab295 100644
--- a/README.md
+++ b/README.md
@@ -352,6 +352,74 @@ Note: The built-in detectors does not require settings. This feature is only for
 
 ---
 
+## Testing
+
+### Running tests
+
+```bash
+composer test
+# or
+./vendor/bin/phpunit
+```
+
+### Snapshot modes
+
+The test suite uses cached HTTP responses and fixtures to avoid network requests during testing. You can control this behavior using environment variables:
+
+| Environment Variable | Description |
+|---------------------|-------------|
+| `UPDATE_EMBED_SNAPSHOTS=1` | Fetch from network and update both cache and fixtures |
+| `EMBED_STRICT_CACHE=1` | Fail if cache or fixture doesn't exist (useful for CI) |
+
+By default (no environment variables set), tests read from cache and generate missing files automatically.
+
+**Note:** If both `UPDATE_EMBED_SNAPSHOTS` and `EMBED_STRICT_CACHE` are set, `UPDATE_EMBED_SNAPSHOTS` takes precedence.
+
+### Cache structure
+
+The test framework uses two types of cached data:
+
+- **Response cache** (`tests/cache/`): Cached HTTP responses from external sites
+- **Fixtures** (`tests/fixtures/`): Expected test results (metadata extracted from cached responses)
+
+### How to update cache
+
+#### When a website changes its HTML structure
+
+If a website updates its HTML and you need to update the cached response and fixture:
+
+```bash
+# Update cache and fixture for a specific test
+UPDATE_EMBED_SNAPSHOTS=1 ./vendor/bin/phpunit --filter testYoutube
+```
+
+#### When adding a new test
+
+After adding a new URL to test:
+
+```bash
+# This will fetch the response and create both cache and fixture
+UPDATE_EMBED_SNAPSHOTS=1 ./vendor/bin/phpunit --filter testNewSite
+```
+
+#### Update all caches at once
+
+To refresh all cached responses and fixtures from the network:
+
+```bash
+UPDATE_EMBED_SNAPSHOTS=1 ./vendor/bin/phpunit
+```
+
+#### For CI environments
+
+Ensure all tests run strictly from cache (fail if any cache is missing):
+
+```bash
+EMBED_STRICT_CACHE=1 ./vendor/bin/phpunit
+```
+
+---
+
 [ico-version]: https://poser.pugx.org/embed/embed/v/stable
 [ico-license]: https://poser.pugx.org/embed/embed/license
 [ico-downloads]: https://poser.pugx.org/embed/embed/downloads
diff --git a/tests/PagesTestCase.php b/tests/PagesTestCase.php
index 7abdb0ff..46db1772 100644
--- a/tests/PagesTestCase.php
+++ b/tests/PagesTestCase.php
@@ -15,7 +15,27 @@
 
 abstract class PagesTestCase extends TestCase
 {
+    /**
+     * Cache mode
+     * -1 = Read from cache (throws an exception if the cache doesn't exist)
+     *  0 = Read from cache (generate the files the first time)
+     *  1 = Read from net without overriding the cache files
+     *  2 = Read from net and override the cache files
+     *
+     * Can be overridden by environment variables:
+     * - UPDATE_EMBED_SNAPSHOTS=1 : Fetch from network and update both cache and fixtures
+     * - EMBED_STRICT_CACHE=1 : Fail if cache or fixture doesn't exist (mode -1)
+     */
     const CACHE = 0;
+
+    /**
+     * Fixtures mode
+     * 0 = Do not override the fixtures
+     * 1 = Override the fixtures
+     *
+     * Can be overridden by environment variable:
+     * - UPDATE_EMBED_SNAPSHOTS=1 : Update both cache and fixtures
+     */
     const FIXTURES = 0;
 
     private const DETECTORS = [
@@ -42,6 +62,45 @@ abstract class PagesTestCase extends TestCase
 
     private static Embed $embed;
 
+    /**
+     * Get cache mode from environment variables or class constant
+     */
+    protected static function getCacheMode(): int
+    {
+        // UPDATE_EMBED_SNAPSHOTS=1 : Fetch from network and override cache
+        if (getenv('UPDATE_EMBED_SNAPSHOTS')) {
+            return 2;
+        }
+
+        // EMBED_STRICT_CACHE=1 : Fail if cache doesn't exist
+        if (getenv('EMBED_STRICT_CACHE')) {
+            return -1;
+        }
+
+        return static::CACHE;
+    }
+
+    /**
+     * Get fixtures mode from environment variable or class constant
+     */
+    protected static function getFixturesMode(): int
+    {
+        // UPDATE_EMBED_SNAPSHOTS=1 : Override fixtures with new results
+        if (getenv('UPDATE_EMBED_SNAPSHOTS')) {
+            return 1;
+        }
+
+        return static::FIXTURES;
+    }
+
+    /**
+     * Check if strict cache mode is enabled
+     */
+    protected static function isStrictMode(): bool
+    {
+        return (bool) getenv('EMBED_STRICT_CACHE');
+    }
+
     private static function getEmbed(): Embed
     {
         if (isset(self::$embed)) {
@@ -49,7 +108,7 @@ private static function getEmbed(): Embed
         }
 
         $dispatcher = new FileClient(__DIR__.'/cache');
-        $dispatcher->setMode(static::CACHE);
+        $dispatcher->setMode(self::getCacheMode());
 
         return self::$embed = new Embed(new Crawler($dispatcher), self::getExtractorFactory());
     }
@@ -63,7 +122,18 @@ protected function assertEmbed(string $url)
         $data = self::getData($extractor);
         $expected = self::readData($uri);
 
-        if (!$expected || static::FIXTURES === 1) {
+        // In strict mode, fail if fixture is missing
+        if (!$expected && self::isStrictMode()) {
+            $this->fail("Fixture not found for {$url}");
+        }
+
+        // Update mode: save fixture and skip
+        if (self::getFixturesMode() === 1) {
+            self::writeData($uri, $data);
+            echo PHP_EOL."Save fixture: {$url}";
+            $this->markTestSkipped('Skipped assertion for '.$url);
+        } elseif (!$expected) {
+            // Missing fixture in non-strict mode: create and skip
             self::writeData($uri, $data);
             echo PHP_EOL."Save fixture: {$url}";
             $this->markTestSkipped('Skipped assertion for '.$url);