Sometimes it does not matter whether you're pointing at a web site or a web page or a part of a web page. Although we already have the url property, it is awkward to use in cases where you also want to talk about the properties of the thing being linked to.

There's also a case to be made that the distinctions between site, page, and part of page are getting blurred, with single page apps loading content dynamically in the background, etc. While it is certainly sometimes useful to distinguish sites from pages and parts of pages, there are occasions when you want to say "there's a chunk of Web content [at url]" that has such-and-so properties.

I think we can address this fairly simply by adding a supertype into the hierarchy between CreativeWork and the trio of WebSite, WebPage, WebContent. The assumption would be that, without further information, the content would typically be expressed in the languages of the browser-oriented Web (HTML/CSS/JS etc.).

We could use encodingFormat or other properties to be more specific about simpler formats, for cases where rendering without a Web platform implementation is relevant.