diff --git a/lib/minify/matthiasmullie-minify/src/CSS.php b/lib/minify/matthiasmullie-minify/src/CSS.php
index e00856f6003..eb98e52ed38 100644
--- a/lib/minify/matthiasmullie-minify/src/CSS.php
+++ b/lib/minify/matthiasmullie-minify/src/CSS.php
@@ -216,7 +216,7 @@ class CSS extends Minify
 
             // grab referenced file & minify it (which may include importing
             // yet other @import statements recursively)
-            $minifier = new static($importPath);
+            $minifier = new self($importPath);
             $minifier->setMaxImportSize($this->maxImportSize);
             $minifier->setImportExtensions($this->importExtensions);
             $importContent = $minifier->execute($source, $parents);
@@ -307,7 +307,8 @@ class CSS extends Minify
              */
             $this->extractStrings();
             $this->stripComments();
-            $this->extractCalcs();
+            $this->extractMath();
+            $this->extractCustomProperties();
             $css = $this->replace($css);
 
             $css = $this->stripWhitespace($css);
@@ -636,35 +637,7 @@ class CSS extends Minify
 
             return $placeholder;
         };
-        // Moodle-specific change MDL-68191 starts.
-        /* This was the old code:
         $this->registerPattern('/\n?\/\*(!|.*?@license|.*?@preserve).*?\*\/\n?/s', $callback);
-        */
-        // This is the new, more accurate and faster regex.
-        $this->registerPattern('/
-            # optional newline
-            \n?
-
-            # start comment
-            \/\*
-
-            # comment content
-            (?:
-                # either starts with an !
-                !
-            |
-                # or, after some number of characters which do not end the comment
-                (?:(?!\*\/).)*?
-
-                # there is either a @license or @preserve tag
-                @(?:license|preserve)
-            )
-
-            # then match to the end of the comment
-            .*?\*\/\n?
-
-            /ixs', $callback);
-        // Moodle-specific change MDL-68191.
 
         $this->registerPattern('/\/\*.*?\*\//s', '');
     }
@@ -706,19 +679,29 @@ class CSS extends Minify
     }
 
     /**
-     * Replace all `calc()` occurrences.
+     * Replace all occurrences of functions that may contain math, where
+     * whitespace around operators needs to be preserved (e.g. calc, clamp)
      */
-    protected function extractCalcs()
+    protected function extractMath()
     {
+        $functions = array('calc', 'clamp', 'min', 'max');
+        $pattern = '/\b('. implode('|', $functions) .')(\(.+?)(?=$|;|})/m';
+
         // PHP only supports $this inside anonymous functions since 5.4
         $minifier = $this;
-        $callback = function ($match) use ($minifier) {
-            $length = strlen($match[1]);
+        $callback = function ($match) use ($minifier, $pattern, &$callback) {
+            $function = $match[1];
+            $length = strlen($match[2]);
             $expr = '';
             $opened = 0;
 
+            // the regular expression for extracting math has 1 significant problem:
+            // it can't determine the correct closing parenthesis...
+            // instead, it'll match a larger portion of code to where it's certain that
+            // the calc() musts have ended, and we'll figure out which is the correct
+            // closing parenthesis here, by counting how many have opened
             for ($i = 0; $i < $length; $i++) {
-                $char = $match[1][$i];
+                $char = $match[2][$i];
                 $expr .= $char;
                 if ($char === '(') {
                     $opened++;
@@ -726,18 +709,41 @@ class CSS extends Minify
                     break;
                 }
             }
-            $rest = str_replace($expr, '', $match[1]);
-            $expr = trim(substr($expr, 1, -1));
 
+            // now that we've figured out where the calc() starts and ends, extract it
             $count = count($minifier->extracted);
-            $placeholder = 'calc('.$count.')';
-            $minifier->extracted[$placeholder] = 'calc('.$expr.')';
+            $placeholder = 'math('.$count.')';
+            $minifier->extracted[$placeholder] = $function.'('.trim(substr($expr, 1, -1)).')';
+
+            // and since we've captured more code than required, we may have some leftover
+            // calc() in here too - go recursive on the remaining but of code to go figure
+            // that out and extract what is needed
+            $rest = str_replace($function.$expr, '', $match[0]);
+            $rest = preg_replace_callback($pattern, $callback, $rest);
 
             return $placeholder.$rest;
         };
 
-        $this->registerPattern('/calc(\(.+?)(?=$|;|}|calc\()/', $callback);
-        $this->registerPattern('/calc(\(.+?)(?=$|;|}|calc\()/m', $callback);
+        $this->registerPattern($pattern, $callback);
+    }
+
+    /**
+     * Replace custom properties, whose values may be used in scenarios where
+     * we wouldn't want them to be minified (e.g. inside calc)
+     */
+    protected function extractCustomProperties()
+    {
+        // PHP only supports $this inside anonymous functions since 5.4
+        $minifier = $this;
+        $this->registerPattern(
+            '/(?<=^|[;}])(--[^:;{}"\'\s]+)\s*:([^;{}]+)/m',
+            function ($match) use ($minifier) {
+                $placeholder = '--custom-'. count($minifier->extracted) . ':0';
+                $minifier->extracted[$placeholder] = $match[1] .':'. trim($match[2]);
+                return $placeholder;
+
+            }
+        );
     }
 
     /**
diff --git a/lib/minify/matthiasmullie-minify/src/JS.php b/lib/minify/matthiasmullie-minify/src/JS.php
index 92389cdd5d0..a0fa649d3a9 100644
--- a/lib/minify/matthiasmullie-minify/src/JS.php
+++ b/lib/minify/matthiasmullie-minify/src/JS.php
@@ -254,7 +254,7 @@ class JS extends Minify
         // of the RegExp methods (a `\` followed by a variable or value is
         // likely part of a division, not a regex)
         $keywords = array('do', 'in', 'new', 'else', 'throw', 'yield', 'delete', 'return',  'typeof');
-        $before = '([=:,;\+\-\*\/\}\(\{\[&\|!]|^|'.implode('|', $keywords).')\s*';
+        $before = '(^|[=:,;\+\-\*\/\}\(\{\[&\|!]|'.implode('|', $keywords).')\s*';
         $propertiesAndMethods = array(
             // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp#Properties_2
             'constructor',
diff --git a/lib/minify/matthiasmullie-minify/src/Minify.php b/lib/minify/matthiasmullie-minify/src/Minify.php
index 3f40bc1578c..4d8dcf40ed6 100644
--- a/lib/minify/matthiasmullie-minify/src/Minify.php
+++ b/lib/minify/matthiasmullie-minify/src/Minify.php
@@ -105,7 +105,7 @@ abstract class Minify
      * @param string|string[] $data
      *
      * @return static
-     * 
+     *
      * @throws IOException
      */
     public function addFile($data /* $data = null, ... */)
@@ -472,7 +472,7 @@ abstract class Minify
      */
     protected function openFileForWriting($path)
     {
-        if (($handler = @fopen($path, 'w')) === false) {
+        if ($path === '' || ($handler = @fopen($path, 'w')) === false) {
             throw new IOException('The file "'.$path.'" could not be opened for writing. Check if PHP has enough permissions.');
         }
 
@@ -490,7 +490,11 @@ abstract class Minify
      */
     protected function writeToFile($handler, $content, $path = '')
     {
-        if (($result = @fwrite($handler, $content)) === false || ($result < strlen($content))) {
+        if (
+            !is_resource($handler) ||
+            ($result = @fwrite($handler, $content)) === false ||
+            ($result < strlen($content))
+        ) {
             throw new IOException('The file "'.$path.'" could not be written to. Check your disk space and file permissions.');
         }
     }