Do not escape characters that do not need escaping in CSS string

src/Value/CSSString.php

@@ -90,8 +90,35 @@ public function getString(): string
      */
     public function render(OutputFormat $outputFormat): string
     {
-        $string = \addslashes($this->string);
+        $string = $this->escape($this->string, $outputFormat->getStringQuotingType());
         $string = \str_replace("\n", '\\A', $string);
         return $outputFormat->getStringQuotingType() . $string . $outputFormat->getStringQuotingType();
     }
+
+    private function escape(string $str, string $quote) : string
+    {
+        $matches = [
+            0x0, // null
+            0x5c, // \
+        ];
+
+        $matches[] = $quote === '"' ? 0x22 : 0x27;
+
+        $length = \strlen($str);
+        $output = '';
+        $previous = 0x0;
+
+        for ($i = 0; $i < $length; ++$i) {
+            $current = \ord($str[$i]);
+
+            if (\in_array($current, $matches, true) && $previous !== 0x5c) {
+                $output .= '\\';
+            }
+
+            $output .= $str[$i];
+            $previous = $current;
+        }
+
+        return $output;
+    }
 }

tests/ParserTest.php

@@ -206,10 +206,10 @@ public function unicodeParsing(): void
                 self::assertSame('"\\"\\""', $firstContentRuleAsString);
             }
             if ($selector === '.test-9') {
-                self::assertSame('"\\"\\\'"', $firstContentRuleAsString);
+                self::assertSame('"\\"\'"', $firstContentRuleAsString);
             }
             if ($selector === '.test-10') {
-                self::assertSame('"\\\'\\\\"', $firstContentRuleAsString);
+                self::assertSame('"\'\\\\"', $firstContentRuleAsString);
             }
             if ($selector === '.test-11') {
                 self::assertSame('"test"', $firstContentRuleAsString);

tests/fixtures/unicode.css

@@ -5,7 +5,7 @@
 .test-6 { content: "\00A5" } /* Same as "¥" */
 .test-7 { content: '\a' } /* Same as "\A" (Newline) */
 .test-8 { content: "\"\22" } /* Same as "\"\"" */
-.test-9 { content: "\"\27" } /* Same as ""\"\'"" */
+.test-9 { content: "\"\27" } /* Same as ""\"'"" */
 .test-10 { content: "\'\\" } /* Same as "'\" */
 .test-11 { content: "\test" } /* Same as "test" */