Topic on Talk:VisualEditor/Design/Software overview

What’s a line, and why is it being introduced here? How is a heading limited to one line, while a paragraph is not? There’s no such concept in HTML.

The idea that an element contains an element or content, but not both, is also novel and in need of basic explanation.

I believe that the first of these is a basic assumption of wikitext; the second is a modelling assumption.

The second bit is mostly a modeling assumption for simplicity. It basically means there is a hierarchy of what can contain what:

• branch nodes can have children that are either branch nodes or content branches, but can't contain content directly. Examples of branch nodes are tables and lists
• content branches can have children, but those children must be content. Examples of content branches are paragraphs, headings and pre's
• content nodes "are" content, and can't have children. Examples are text nodes (plain or annotated text), images and br's

This means that some things that are legal in HTML are not legal in our model. For instance, in the HTML that we get, it's common for <li>'s to contain text directly. In our model, that's represented as a list item containing a paragraph containing a text node.