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A DOM comment node stores comment character data in the Document Object Model. In HTML source, a normal comment can be written as <!-- note -->; parsing it produces a comment node rather than a DOM element or a DOM text node containing visible words.^[ref][ref]

For example, <p>A<!-- note -->B</p> gives the paragraph three child DOM nodes: a text node containing A, a comment node, and a text node containing B. Its childNodes.length is three, while its element-only children.length is zero. Its textContent is AB: the comment does not contribute to this descendant-text result.^[ref]

A traversal over every child can consequently encounter nodes absent from an element-only traversal. A DOM comment node can be inserted, moved or removed while editing the DOM tree, although it produces no ordinary displayed text. Removing the comment in this example leaves two adjacent text nodes; adjacency alone does not merge their object identities.

Comments are not a way to keep information secret from someone receiving a document: their data remains in the supplied source or document model. When generating comments, obey HTML's syntax rather than assuming arbitrary text can always be placed between the delimiters.

A DOM node is an object participating in the node model of the Document Object Model. Documents, DOM elements, DOM text nodes, DOM comment nodes and DOM document fragments are different node kinds. They share operations and relationships defined by the Node interface, while particular kinds expose additional data and constraints.^[ref]

For example, a paragraph element can contain a text node. The paragraph has a tag name and attributes; its text child stores character data. Both are DOM nodes, but an element-specific operation such as setting an attribute is not thereby available on the text node.

Insertion also concerns object identity. Suppose an existing paragraph is a child of element A. Executing B.appendChild(paragraph) moves that same node under B, removing it from A's child list. It does not automatically make a second paragraph. Cloning is a separate operation, with its own treatment of descendants.^[ref]

A newly created node can exist without being connected to a displayed document. Conversely, a connected node need not produce a visible box: a CSS declaration can suppress its display. A reference to a node therefore identifies a document-model object, not a promise about visibility or a fixed screen position. Reading the node kind and its current tree relationships is the appropriate starting point for manipulation.

A DOM tree is an ordered parent-child structure of DOM nodes in the Document Object Model. Children have an order, each node has at most one parent in a tree, and following parent relationships cannot form a cycle. A document tree is rooted at a document node; its document element is a different node within that structure.^[ref]

In <p>Hello <em>world</em>.</p>, the paragraph has three children: a text node containing Hello and a following space, the emphasis element, and a text node containing the period. The emphasis element has its own text child containing world. The word world is therefore a descendant of the paragraph, but not its direct child.

That distinction explains selector behavior. A child CSS combinator tests an immediate relationship, while a descendant combinator can follow multiple parent-child steps. Inspecting only element children also gives a different list from inspecting every child node, because DOM text nodes and DOM comment nodes are nodes too.

The DOM tree describes document structure rather than the exact source spelling or the boxes painted on screen. HTML parsing can create structure from omitted tags, and CSS can change rendering without inserting a document child. Attribute data is attached to elements under its specified model; it is not simply another ordinary child in the document's child list.^[ref]