This HTML applies all common transforms for general API clients that can make use of structured APIs.
By "content", this means the HTML that is specifically a pages content, as opposed to metadata about an article that is typically encoded within the page itself.
A good example of this is references. These are typically included as a long list at the bottom of an page. These will be omitted from this API and instead clients can obtain them by using the structured JSON API.
Other examples include hatnotes, page issues and categories.
Additionally, the transforms may be applied to improve rendering/readability (such as moving the lead paragraph, inserting Wikidata descriptions, or lead images). A list of the transforms that are applied at this layer is TBD (pending outcome of understanding common transforms across all clients).