diff --git a/core/lib/Drupal/Core/Asset/CssOptimizer.php b/core/lib/Drupal/Core/Asset/CssOptimizer.php
index 87575e0..99fdbc1 100644
--- a/core/lib/Drupal/Core/Asset/CssOptimizer.php
+++ b/core/lib/Drupal/Core/Asset/CssOptimizer.php
@@ -205,8 +205,10 @@ protected function processCss($contents, $optimize = FALSE) {
       // whitespace.
       // @see http://php.net/manual/regexp.reference.subpatterns.php
       $contents = preg_replace('<
+        # Do not strip any space from within single or double quotes
+          ('.$double_quot.'|'.$single_quot.')
         # Strip leading and trailing whitespace.
-          \s*([@{};,])\s*
+        | \s*([@{};,])\s*
         # Strip only leading whitespace from:
         # - Closing parenthesis: Retain "@media (bar) and foo".
         | \s+([\)])
@@ -214,11 +216,11 @@ protected function processCss($contents, $optimize = FALSE) {
         # - Opening parenthesis: Retain "@media (bar) and foo".
         # - Colon: Retain :pseudo-selectors.
         | ([\(:])\s+
-      >xS',
-        // Only one of the three capturing groups will match, so its reference
+      >xSs',
+        // Only one of the four capturing groups will match, so its reference
         // will contain the wanted value and the references for the
         // two non-matching groups will be replaced with empty strings.
-        '$1$2$3',
+        '$1$2$3$4',
         $contents
       );
       // End the file with a new line.
diff --git a/core/tests/Drupal/Tests/Core/Asset/css_test_files/quotes.css b/core/tests/Drupal/Tests/Core/Asset/css_test_files/quotes.css
new file mode 100644
index 0000000..3fe0f7d
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/Asset/css_test_files/quotes.css
@@ -0,0 +1,31 @@
+/* Example from https://www.w3.org/TR/CSS2/syndata.html#rule-sets */
+q[example="public class foo\
+{\
+    private int x;\
+\
+    foo(int x) {\
+        this.x = x;\
+    }\
+\
+}"] { color: red }
+
+/* A pseudo selector with essential whitespace wrapped in quotes. */
+q[style*="quotes: none"] {
+    quotes: none;
+}
+
+q[style*='quotes: none'] {
+    quotes: none;
+}
+
+q:after {
+    content: ": colon & escaped double \" quotes \".";
+}
+
+q:after {
+    content: ' (brackets & escaped single \' quotes \') ';
+}
+
+q:after {
+    content: "I'm Quote";
+}
diff --git a/core/tests/Drupal/Tests/Core/Asset/css_test_files/quotes.css.optimized.css b/core/tests/Drupal/Tests/Core/Asset/css_test_files/quotes.css.optimized.css
new file mode 100644
index 0000000..716cef7
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/Asset/css_test_files/quotes.css.optimized.css
@@ -0,0 +1,9 @@
+q[example="public class foo\
+{\
+    private int x;\
+\
+    foo(int x) {\
+        this.x = x;\
+    }\
+\
+}"]{color:red}q[style*="quotes: none"]{quotes:none;}q[style*='quotes: none']{quotes:none;}q:after{content:": colon & escaped double \" quotes \".";}q:after{content:' (brackets & escaped single \' quotes \') ';}q:after{content:"I'm Quote";}