diff --git a/src/wp-includes/formatting.php b/src/wp-includes/formatting.php
index 0e175f24e7..2d37d7f992 100644
--- a/src/wp-includes/formatting.php
+++ b/src/wp-includes/formatting.php
@@ -2058,8 +2058,10 @@ function sanitize_sql_orderby( $orderby ) {
 /**
  * Sanitizes an HTML classname to ensure it only contains valid characters.
  *
- * Strips the string down to A-Z,a-z,0-9,_,-. If this results in an empty
- * string then it will return the alternative value supplied.
+ * Strips the string down to A-Z,a-z,0-9,_,- and attempts to further adhere to
+ * the specification (@see https://www.w3.org/TR/CSS22/syndata.html#characters).
+ * If this results in an empty string then it will return the alternative
+ * value supplied.
  *
  * @todo Expand to support the full range of CDATA that a class attribute can contain.
  *
@@ -2077,6 +2079,16 @@ function sanitize_html_class( $class, $fallback = '' ) {
 	//Limit to A-Z,a-z,0-9,_,-
 	$sanitized = preg_replace( '/[^A-Za-z0-9_-]/', '', $sanitized );
 
+	//Replace digits, hyphen and double hyphen at begining of class name because
+	//they are not allowed as per spec.
+	$sanitized = preg_replace( array(
+		'/^[0-9]/',
+		'/^(-[0-9])|^(--)/',
+	), array(
+		'_',
+		'__',
+	), $sanitized);
+
 	if ( '' == $sanitized && $fallback ) {
 		return sanitize_html_class( $fallback );
 	}
diff --git a/tests/phpunit/tests/formatting/SanitizeClassName.php b/tests/phpunit/tests/formatting/SanitizeClassName.php
new file mode 100644
index 0000000000..970059bba9
--- /dev/null
+++ b/tests/phpunit/tests/formatting/SanitizeClassName.php
@@ -0,0 +1,21 @@
+<?php
+
+/**
+ * @group formatting
+ */
+class Tests_Formatting_SanitizeClassName extends WP_UnitTestCase {
+	function test_html_class_name() {
+		$tests = array(
+			'space separated' => 'spaceseparated',
+			'CASE_PRESERVED' => 'CASE_PRESERVED',
+			'$invalid^chars' => 'invalidchars',
+			'0starts-with-integer' => '_starts-with-integer',
+			'-starts-with--' => '-starts-with--', // this is allowed because the rule is /^-[0-9]/
+			'-0starts-with-integer' => '__starts-with-integer',
+			'--starts-with---' => '__starts-with---',
+		);
+		foreach ( $tests as $key => $value ) {
+			$this->assertEquals( $value, sanitize_html_class( $key ) );
+		}
+	}
+}