A hidden paragraph:
The hidden attribute is a boolean attribute.
When present, it specifies that an element is not yet, or is no longer, relevant.
Browsers should not display elements that have the hidden attribute specified.
The numbers in the table specify the first browser version that fully supports the attribute.
The hidden attribute is new in HTML5.
In XHTML, attribute minimization is forbidden, and the hidden attribute must be defined as <element hidden="hidden">.
HTML Tutorial: HTML Attributes
CSS Tutorial: CSS Display and Visibility
CSS Reference: CSS display Property
CSS Reference: CSS visibility Property