Fix the issues reported by phpcs

-          $group_keys = $item['preprocess'] ? [$item['type'], $item['group'], $item['media'], $item['browsers']] : FALSE;
+          $group_keys = $item['preprocess'] ? [$item['type'], $item['group'], $item['media'],
+            $item['browsers'],
+          ] : FALSE;

Lines are allowed to be longer than 80 characters, if the code is more readable.

-.test1 { display:block;}
+A large comment block to test for segfaults and speed. This is 60K a's. Extreme but useful to demonstrate flaws in comment striping regexp. */

Where are the 60,000 As? That comment contains 60,000 As for a reason, and that reason is even explained from that comment: Extreme but useful to demonstrate flaws in comment striping regexp. That comment cannot be changed.

@@ -21,12 +20,9 @@ body {
  * CSS spec says that all whitespace is valid whitespace, so this selector
  * should be just as good as the one above.
  */
-.this
-.is
-.a
-.test {
-font: 1em/100% Verdana, sans-serif;
-color: #494949;
+.this .is .a .test {
+  font: 1em/100% Verdana, sans-serif;
+  color: #494949;

That is a test file. It is used to test something and it cannot be changed.

diff --git a/advagg_css_minify/tests/src/Kernel/Asset/css_test_files/css_input_without_import.css.optimized.css b/advagg_css_minify/tests/src/Kernel/Asset/css_test_files/css_input_without_import.css.optimized.css
index ebe82fb..bdc01fd 100755
--- a/advagg_css_minify/tests/src/Kernel/Asset/css_test_files/css_input_without_import.css.optimized.css
+++ b/advagg_css_minify/tests/src/Kernel/Asset/css_test_files/css_input_without_import.css.optimized.css
@@ -1 +1,43 @@
-body{margin:0;padding:0;background:#edf5fa;font:76%/170% Verdana,sans-serif;color:#494949}.this .is .a .test{font:1em/100% Verdana,sans-serif;color:#494949}.this .is .a .test{font:1em/100% Verdana,sans-serif;color:#494949}some :pseudo .thing{border-radius:3px}::-moz-selection{background:#000;color:#fff}::selection{background:#000;color:#fff}@media print{*{background:#000 !important;color:#fff !important}@page{margin:.5cm}}@media screen and (max-device-width:480px){background:#000;color:#fff}textarea,select{font:1em/160% Verdana,sans-serif;color:#494949}
\ No newline at end of file
+body {
+  margin: 0;
+  padding: 0;
+  background: #edf5fa;
+  font: 76%/170% Verdana,sans-serif;
+  color: #494949;
+}
+.this .is .a .test {
+  font: 1em/100% Verdana,sans-serif;
+  color: #494949;
+}
+.this .is .a .test {
+  font: 1em/100% Verdana,sans-serif;
+  color: #494949;
+}
+some :pseudo .thing {
+  border-radius: 3px;
+}
+::-moz-selection {
+  background: #000;
+  color: #fff;
+}
+::selection {
+  background: #000;
+  color: #fff;
+}
+@media print {
+  * {
+    background: #000 !important;
+    color: #fff !important;
+  }
+  @page {
+    margin: .5cm;
+  }
+}
+@media screen and (max-device-width:480px) {
+  background: #000;
+  color: #fff;
+}
+textarea select {
+  font: 1em/160% Verdana,sans-serif;
+  color: #494949;
+}

The css_test_files directory probably contain test files that need to be preserved as they are.

I applied patch in #22 and it applied cleanly

-     headers for all optimized files. Only supported on Edge 15+ and Firefox 49+.
+     headers for all optimized files.
+     Only supported on Edge 15+ and Firefox 49+.

Making a line shorter than 81 characters does not mean starting a new sentence on a new line.

-        // Only delete files older than stale file threshold defined under Cron Options.
+        // Only delete files older than stale
+        // file threshold defined under Cron Options.

Lines should as closer as possible to 80 characters, without splitting words on two lines. That does not mean moving six words on a new line.

-      '#options' => ['google' => 'Google', 'microsoft' => 'Microsoft'],
+      '#options' => [$this->t('google' => 'Google'),
+        $this->t('microsoft' => 'Microsoft'),
+      ],

The changed code contains syntax errors.

-    $contents = preg_replace_callback('/@import\s*(?:url\(\s*)?[\'"]?(?![a-z]+:)(?!\/\/)([^\'"\()]+)[\'"]?\s*\)?\s*;/', [$this, 'loadNestedFile'], $contents);
+    $contents = preg_replace_callback('/@import\s*(?:url\(\s*)?[\'"]?(?![a-z]+:)(?!\/\/)([^\'"\()]+)[\'"]?\s*\)?\s*;/',
+    [$this, 'loadNestedFile'], $contents);
 

Code lines are allowed to exceed 80 characters if they are easier to understand. The changed code is not correctly formatted.

-@charset "UTF-8";
-html{font-family:"sans-serif";}
+@charset "UTF-8"; html {
+  font-family: "sans-serif";
+}

The @charset "UTF-8"; line is probably not related to the following CSS style to be placed on the same line.

-    $response->setContent(preg_replace_callback($pattern, [$this, 'forceAbsolutePathsCallback'], $content));
+    $response->setContent(preg_replace_callback($pattern,
+    [
+      $this,
+      'forceAbsolutePathsCallback',
+    ],
+    $content));
 

The new code is not correctly formatted.

     $cookie_name = 'AdvAggDisabled';
     $key = Crypt::hashBase64($this->privateKey->get());
+    $stack = $stack->getCurrentRequest();

$stack has not been initialized; it is not possible to call $stack->getCurrentRequest()