Merge branch '3.0' into 3.1

diff --cc wcfsetup/install/files/lib/system/html/input/filter/MessageHtmlInputFilter.class.php
index 3d5bdcf516864fec774c6b9daba87ab6b17eb183,a56d07032ee7ad5a70c01d053ab620fd5816182c..b53ea295b459266d473236a05f5b068bd74c23e9
--- 1/wcfsetup/install/files/lib/system/html/input/filter/MessageHtmlInputFilter.class.php
--- 2/wcfsetup/install/files/lib/system/html/input/filter/MessageHtmlInputFilter.class.php
+++ b/wcfsetup/install/files/lib/system/html/input/filter/MessageHtmlInputFilter.class.php
@@@ -53,9 -52,9 +53,10 @@@ class MessageHtmlInputFilter implement
                        
                        $config->set('CSS.AllowedProperties', ['color', 'font-family', 'font-size']);
                        $config->set('HTML.ForbiddenAttributes', ['*@lang', '*@xml:lang']);
+                       $config->set('HTML.Doctype', 'HTML 4.01 Strict');
                        
                        $allowedSchemes = $config->get('URI.AllowedSchemes');
 +                      $allowedSchemes['steam'] = true;
                        $allowedSchemes['ts3server'] = true;
                        $config->set('URI.AllowedSchemes', $allowedSchemes);