The save, close, etc... buttons don't work if you put "<center>" in the Extended elements in editor parameters Seems a js error. This is the console log of chrome:

editor.min.js?5b5fdff1570ce4195ab4eac77f9376bb:2 Uncaught DOMException: Failed to execute 'querySelectorAll' on 'Element': 'optgroup,select,datalist,option,isindex,menu,noscript,dd,dt,dl,caption,ul,ol,li,td,tr,th,tfoot,thead,tbody,table,hr,optgroup,select,datalist,option,isindex,menu,noscript,dd,dt,dl,caption,ul,ol,li,td,tr,th,tfoot,thead,tbody,table,hr,figure,nav,aside,hgroup,section,article,footer,header,fieldset,dir,center,blockquote,form,pre,address,div,p,h6,h5,h4,h3,h2,h1,figure,nav,aside,hgroup,section,article,footer,header,fieldset,dir,center,blockquote,form,pre,address,div,p,h6,h5,h4,h3,h2,h1,<center>,<center>,mce-comment,mce-comment' is not a valid selector.
    at Object.getActiveLine (/components/com_jce/editor/libraries/js/editor.min.js?5b5fdff1570ce4195ab4eac77f9376bb:2:11481)
    at Editor.<anonymous> (/components/com_jce/editor/libraries/js/editor.min.js?5b5fdff1570ce4195ab4eac77f9376bb:2:5763)
    at Dispatcher.dispatch (/components/com_jce/editor/tiny_mce/tiny_mce.js?5b5fdff1570ce4195ab4eac77f9376bb:7:6615)
    at Editor.save (/components/com_jce/editor/tiny_mce/tiny_mce.js?5b5fdff1570ce4195ab4eac77f9376bb:15:17721)
    at Object.getContent (/components/com_jce/editor/libraries/js/editor.min.js?5b5fdff1570ce4195ab4eac77f9376bb:2:16056)
    at Object.Joomla.submitbutton (/administrator/index.php?option=com_content&view=article&layout=edit&id=3089:150:13)
    at HTMLButtonElement.onclick (/administrator/index.php?option=com_content&view=article&layout=edit&id=3089:796:102)
getActiveLine @ editor.min.js?5b5fdff1570ce4195ab4eac77f9376bb:2
(anonymous) @ editor.min.js?5b5fdff1570ce4195ab4eac77f9376bb:2
dispatch @ tiny_mce.js?5b5fdff1570ce4195ab4eac77f9376bb:7
save @ tiny_mce.js?5b5fdff1570ce4195ab4eac77f9376bb:15
getContent @ editor.min.js?5b5fdff1570ce4195ab4eac77f9376bb:2
Joomla.submitbutton @ index.php?option=com_content&view=article&layout=edit&id=3089:150
onclick @ index.php?option=com_content&view=article&layout=edit&id=3089:796

The error happens in both, frontend & backend and the error doesn't appear when I change the editor to tinymce I have tried version 2.8.8, 2.8.9 & 2.8.10 rc2 with the same results If I erase the <center> element i works well :)

Yes you are right,I know.
Only that maybe will be better to alert the user, or strip the <>, than fail to save or close in the edit. no?