diff --git a/classes/errors.php b/classes/errors.php
index 31be558cf..aa626d017 100644
--- a/classes/errors.php
+++ b/classes/errors.php
@@ -14,4 +14,27 @@ class Errors {
 	static function to_json(string $code, array $params = []): string {
 		return json_encode(["error" => ["code" => $code, "params" => $params]]);
 	}
+
+	static function libxml_last_error() : string {
+		$error = libxml_get_last_error();
+		$error_formatted = "";
+
+		if ($error) {
+			foreach (libxml_get_errors() as $error) {
+				if ($error->level == LIBXML_ERR_FATAL) {
+					// currently only the first error is reported
+					$error_formatted = self::format_libxml_error($error);
+					break;
+				}
+			}
+		}
+
+		return UConverter::transcode($error_formatted, 'UTF-8', 'UTF-8');
+	}
+
+	static function format_libxml_error(LibXMLError $error) : string {
+		return sprintf("LibXML error %s at line %d (column %d): %s",
+			$error->code, $error->line, $error->column,
+			$error->message);
+	}
 }
diff --git a/classes/feedparser.php b/classes/feedparser.php
index 6ce69cc89..3ed0647d2 100644
--- a/classes/feedparser.php
+++ b/classes/feedparser.php
@@ -193,10 +193,9 @@ class FeedParser {
 		}
 	}
 
+	/** @deprecated use Errors::format_libxml_error() instead */
 	function format_error(LibXMLError $error) : string {
-		return sprintf("LibXML error %s at line %d (column %d): %s",
-			$error->code, $error->line, $error->column,
-			$error->message);
+		return Errors::format_libxml_error($error);
 	}
 
 	// libxml may have invalid unicode data in error messages
diff --git a/classes/feeds.php b/classes/feeds.php
index 65027cb9e..2c37d659a 100755
--- a/classes/feeds.php
+++ b/classes/feeds.php
@@ -254,6 +254,10 @@ class Feeds extends Handler_Protected {
 
 								$line["buttons_left"] .= $button_doc->saveXML($button_doc->firstChild);
 							}
+						} else if ($result) {
+							user_error(get_class($plugin) .
+								" plugin: content provided in HOOK_ARTICLE_LEFT_BUTTON is not valid XML: " .
+								Errors::libxml_last_error() . " $result", E_USER_WARNING);
 						}
 					},
 					$line);
@@ -273,6 +277,10 @@ class Feeds extends Handler_Protected {
 
 								$line["buttons"] .= $button_doc->saveXML($button_doc->firstChild);
 							}
+						} else if ($result) {
+							user_error(get_class($plugin) .
+								" plugin: content provided in HOOK_ARTICLE_BUTTON is not valid XML: " .
+								Errors::libxml_last_error() . " $result", E_USER_WARNING);
 						}
 					},
 					$line);
diff --git a/classes/plugin.php b/classes/plugin.php
index 3bced3b04..39af6a9a1 100644
--- a/classes/plugin.php
+++ b/classes/plugin.php
@@ -98,7 +98,7 @@ abstract class Plugin {
 
 	/* GLOBAL hooks are invoked in global context, only available to system plugins (loaded via .env for all users) */
 
-	/** Adds buttons for article (on the right) - e.g. mail, share, add note.
+	/** Adds buttons for article (on the right) - e.g. mail, share, add note. Generated markup must be valid XML.
 	 * @param array<string,mixed> $line
 	 * @return string
 	 * @see PluginHost::HOOK_ARTICLE_BUTTON
@@ -307,7 +307,7 @@ abstract class Plugin {
 		return [];
 	}
 
-	/** Adds per-article buttons on the left side
+	/** Adds per-article buttons on the left side. Generated markup must be valid XML.
 	 * @param array<string,mixed> $row
 	 * @return string
 	 * @see PluginHost::HOOK_ARTICLE_LEFT_BUTTON