diff --git a/src/markup/engine/remarkup/blockrule/PhutilRemarkupCodeBlockRule.php b/src/markup/engine/remarkup/blockrule/PhutilRemarkupCodeBlockRule.php
index b91f818..3be2018 100644
--- a/src/markup/engine/remarkup/blockrule/PhutilRemarkupCodeBlockRule.php
+++ b/src/markup/engine/remarkup/blockrule/PhutilRemarkupCodeBlockRule.php
@@ -1,223 +1,228 @@
 <?php
 
 final class PhutilRemarkupCodeBlockRule extends PhutilRemarkupBlockRule {
 
   public function getMatchingLineCount(array $lines, $cursor) {
     $num_lines = 0;
     $match_ticks = null;
     if (preg_match('/^(\s{2,}).+/', $lines[$cursor])) {
       $match_ticks = false;
     } else if (preg_match('/^\s*(```)/', $lines[$cursor])) {
       $match_ticks = true;
     } else {
       return $num_lines;
     }
 
     $num_lines++;
 
     if ($match_ticks &&
         preg_match('/^\s*(```)(.*)(```)\s*$/', $lines[$cursor])) {
       return $num_lines;
     }
 
     $cursor++;
 
     while (isset($lines[$cursor])) {
       if ($match_ticks) {
         if (preg_match('/```\s*$/', $lines[$cursor])) {
           $num_lines++;
           break;
         }
         $num_lines++;
       } else {
         if (strlen(trim($lines[$cursor]))) {
           if (!preg_match('/^\s{2,}/', $lines[$cursor])) {
             break;
           }
         }
         $num_lines++;
       }
       $cursor++;
     }
 
     return $num_lines;
   }
 
   public function markupText($text, $children) {
     if (preg_match('/^\s*```/', $text)) {
       // If this is a ```-style block, trim off the backticks and any leading
       // blank line.
       $text = preg_replace('/^\s*```(\s*\n)?/', '', $text);
       $text = preg_replace('/```\s*$/', '', $text);
     }
 
     $lines = explode("\n", $text);
     while ($lines && !strlen(last($lines))) {
       unset($lines[last_key($lines)]);
     }
 
     $options = array(
       'counterexample'  => false,
       'lang'            => null,
       'name'            => null,
       'lines'           => null,
     );
 
     $parser = new PhutilSimpleOptions();
     $custom = $parser->parse(head($lines));
     if ($custom) {
       $valid = true;
       foreach ($custom as $key => $value) {
         if (!array_key_exists($key, $options)) {
           $valid = false;
           break;
         }
       }
       if ($valid) {
         array_shift($lines);
         $options = $custom + $options;
       }
     }
 
     // Normalize the text back to a 0-level indent.
     $min_indent = 80;
     foreach ($lines as $line) {
       for ($ii = 0; $ii < strlen($line); $ii++) {
         if ($line[$ii] != ' ') {
           $min_indent = min($ii, $min_indent);
           break;
         }
       }
     }
 
     $text = implode("\n", $lines);
     if ($min_indent) {
       $indent_string = str_repeat(' ', $min_indent);
       $text = preg_replace('/^'.$indent_string.'/m', '', $text);
     }
 
     if ($this->getEngine()->isTextMode()) {
       $out = array();
 
       $header = array();
       if ($options['counterexample']) {
         $header[] = 'counterexample';
       }
       if ($options['name'] != '') {
         $header[] = 'name='.$options['name'];
       }
       if ($header) {
         $out[] = implode(', ', $header);
       }
 
       $text = preg_replace('/^/m', '  ', $text);
       $out[] = $text;
 
       return implode("\n", $out);
     }
 
     if (empty($options['lang'])) {
       // If the user hasn't specified "lang=..." explicitly, try to guess the
       // language. If we fail, fall back to configured defaults.
       $lang = PhutilLanguageGuesser::guessLanguage($text);
       if (!$lang) {
         $lang = nonempty(
           $this->getEngine()->getConfig('phutil.codeblock.language-default'),
           'text');
       }
       $options['lang'] = $lang;
     }
 
     $code_body = $this->highlightSource($text, $options);
 
     $name_header = null;
     if ($this->getEngine()->isHTMLMailMode()) {
       $header_attributes = array (
         'style' => 'padding: 6px 8px;
           background: #fdf5d4;
           color: rgba(0,0,0,.75);
           font-weight: bold;
           display: inline-block;
           border-top: 1px solid #f1c40f;
           border-left: 1px solid #f1c40f;
           border-right: 1px solid #f1c40f;
           margin-bottom: -1px;',
       );
     } else {
       $header_attributes = array(
         'class' => 'remarkup-code-header',
       );
     }
     if ($options['name']) {
       $name_header = phutil_tag(
         'div',
         $header_attributes,
         $options['name']);
     }
 
+    $class = 'remarkup-code-block';
+    if ($options['counterexample']) {
+      $class = 'remarkup-code-block code-block-counterexample';
+    }
+
     $attributes = array(
-        'class' => 'remarkup-code-block',
+        'class' => $class,
         'data-code-lang' => $options['lang'],
         'data-sigil' => 'remarkup-code-block',
     );
 
     return phutil_tag(
       'div',
       $attributes,
       array($name_header, $code_body));
   }
 
   private function highlightSource($text, array $options) {
     if ($options['counterexample']) {
       $aux_class = ' remarkup-counterexample';
     } else {
       $aux_class = null;
     }
 
     $aux_style = null;
 
     if ($this->getEngine()->isHTMLMailMode()) {
       if ($options['counterexample']) {
         $aux_style = 'border: 1px solid #c0392b;
           background: #f4dddb;
           font-size: 10x;
           padding: 8px;';
       } else {
         $aux_style = 'border: 1px solid #f1c40f;
           background: #fdf5d4;
           font-size: 10x;
           padding: 8px;';
       }
     }
 
     if ($options['lines']) {
       // Put a minimum size on this because the scrollbar is otherwise
       // unusable.
       $height = max(6, (int)$options['lines']);
       $aux_style = $aux_style
         .' '
         .'max-height: '
         .(2 * $height)
         .'em; overflow: auto;';
     }
 
     $engine = $this->getEngine()->getConfig('syntax-highlighter.engine');
     if (!$engine) {
       $engine = 'PhutilDefaultSyntaxHighlighterEngine';
     }
     $engine = newv($engine, array());
     $engine->setConfig(
       'pygments.enabled',
       $this->getEngine()->getConfig('pygments.enabled'));
     return phutil_tag(
       'pre',
       array(
         'class' => 'remarkup-code'.$aux_class,
         'style' => $aux_style,
       ),
       PhutilSafeHTML::applyFunction(
         'rtrim',
         $engine->highlightSource($options['lang'], $text)));
   }
 
 }