for issue #37504

markdown_absolute_url.patch

diff --git markdown.php markdown.php
index ed79197..0cde939 100644
--- markdown.php
+++ markdown.php
@@ -761,6 +761,17 @@ class Markdown_Parser {
 
 		$url = $this->encodeAttribute($url);
 
+		// For some links like [xxx](/node/xxx), turn them into absolute ones
+		// so when they're contained in feeds, the W3C feed validator won't
+		// complain about the relative urls.
+		// @see issue #37504
+		if (strpos($url, '/') === 0) {
+			// Remove the leading slash
+			$url = ltrim($url, '/');
+			// Use drupal's url() to generate an absolute url
+			$url = url($url, array('absolute' => TRUE));
+		}
+
 		$result = "<a href=\"$url\"";
 		if (isset($title)) {
 			$title = $this->encodeAttribute($title);