The first stage of WikiText processing is the parser. A Parser is provided by a module with module-type: parser and is responsible to transform block of text to a parse-tree. The parse-tree consists of nested nodes like

{type: "element", tag: <string>, attributes: {}, children: []} - an HTML element
{type: "text", text: <string>} - a text node
{type: "entity", entity: <string>} - an HTML entity like &copy; for a copyright symbol
{type: "raw", html: <string>} - raw HTML

The core plug-in provides a recursive descent WikiText parser which loads it's individual rules from individual modules. Thus a developer can provide additional rules by using module-type: wikirule. Each rule can produce a list of parse-tree nodes. A simple example for a wikirule producing a <hr> from --- can be found in horizrule.js

HTML tags can be embedded into WikiText because of the html rule. This rule matches HTML tag syntax and creates type: "element" nodes. But the html-rule has another special purpose. By parsing the HTML tag syntax it implicitly parses WikiText widgets. It the recognises them by the $ character at the beginning of the tag name and instead of producing "element" nodes it uses the tag name for the type:

{type: "list", tag: "$list", attributes: {}, children: []} - a list element

The Widgets part will reveal why this makes sense and how each node is transformed into a widget. Another special characteristic of the html-rule or the parse nodes in general is the attributes property. Attributes in the parse-tree are not stored as simple strings but they are nodes of its own to make indirect text references available as attributes as described in Widgets:

{type: "string", value: <string>} - literal string
{type: "indirect", textReference: <textReference>} - indirect through a text reference