Get this error after SAVE on any Content > Block.
Temporarily allowed to save HTML value that contains restricted elements. Allowed HTML tags are: div, a, p, span, em, strong, ul, li, ol, h5, h4, h3, h2, h1, table, tbody, tr, td, th, tfoot, img, hr, figure, button, i, u, br, b
@ShebaThis is not an error, it is a warning. When you read it again you'll see that It is giving you a message that you have used a HTML tag which is not in allowed list.
This could be a feature Magento is working on or something else not sure of...