Если CSS свойство display перекрывает атрибут hidden, то нафига последний нужен? Ну напишу я display: flex и атрибут hidden и?..

All HTML elements may have the hidden content attribute set. The hidden attribute is a boolean attribute. When specified on an element, it indicates that the element is not yet, or is no longer, directly relevant to the page’s current state, or that it is being used to declare content to be reused by other parts of the page as opposed to being directly accessed by the user. User agents should not render elements that have the hidden attribute specified.

Should not render, ага. Наркоманы.