Indicates that the style extensions ([style](extension-rendering-style.html), [markdown](extension-rendering-markdown.html)) and [xhtml](extension-rendering-xhtml.html)) in this resource are essential to the interpretation of the styled text and that it cannot be rendered without the style without losing essential information.
A quality common to all members of an object class. A property may be any feature that humans naturally use to distinguish one individual object from another. It is the human perception of a single quality of an object class in the real world. It is conceptual and thus has no particular associated means of representation by which the property can be communicated.