Every HTML element is a member of one or more content categories — these categories group elements that share common characteristics. This is a loose grouping (it doesn't actually create a relationship among elements of these categories), but they help define and describe the categories' shared behavior and their associated rules, especially when you come upon their intricate details. It's also possible for elements to not be a member of any of these categories.
There are three types of content categories:
- Main content categories, which describe common rules shared by many elements.
- Form-related content categories, which describe rules common to form-related elements.
- Specific content categories, which describe rare categories shared only by a few elements, sometimes only in a specific context.
Embedded content imports another resource or inserts content from another mark-up language or namespace into the document.
Elements belonging to the flow content category typically contain text or embedded content.
Form-associated content comprises elements that have a form owner, exposed by a form attribute. A form owner is either the containing
Heading content defines the title of a section, whether marked by an explicit sectioning content element, or implicitly defined by the heading content itself.
Interactive content includes elements that are specifically designed for user interaction.
Elements that can be associated with
Elements that are listed in the form.elements and fieldset.elements IDL collections.
Elements belonging to the metadata content category modify the presentation or the behavior of the rest of the document, set up links to other documents, or convey other out of band information.
Content is palpable when it's neither empty or hidden; it is content that is rendered and is substantive. Elements whose model is flow content or phrasing content should have at least one node which is palpable.
Phrasing content defines the text and the mark-up it contains. Runs of phrasing content make up paragraphs.
Elements that can be affected when a form is reset.
Script-supporting elements are elements which don't directly contribute to the rendered output of a document. Instead, they serve to support scripts, either by containing or specifying script code directly, or by specifying data that will be used by scripts.
Elements belonging to the sectioning content model create a section in the current outline that defines the scope of
Elements that can be used for constructing the form data set when the form is submitted.