diff --git a/src/htmlStringToDocument.ts b/src/htmlStringToDocument.ts
new file mode 100644
index 0000000..30244cc
--- /dev/null
+++ b/src/htmlStringToDocument.ts
@@ -0,0 +1,233 @@
+import { parseHtml } from "./parseHtml";
+import {
+  BLOCKS,
+  NodeData,
+  TopLevelBlock,
+  Document,
+  MARKS,
+  INLINES,
+  Node,
+  Text,
+  Block,
+  Inline,
+  Mark,
+} from "@contentful/rich-text-types";
+import { HTMLElementNode, HTMLNode, HTMLTagName } from "./types";
+
+const BLOCK_TYPES = Object.values(BLOCKS);
+const INLINE_TYPES = Object.values(INLINES);
+const MARK_TYPES = Object.values(MARKS);
+
+const isBlockType = (nodeType: BLOCKS | MARKS | INLINES): nodeType is BLOCKS =>
+  BLOCK_TYPES.includes(nodeType as BLOCKS);
+const isInlineType = (
+  nodeType: BLOCKS | MARKS | INLINES
+): nodeType is INLINES => INLINE_TYPES.includes(nodeType as INLINES);
+const isMarkType = (nodeType: BLOCKS | MARKS | INLINES): nodeType is MARKS =>
+  MARK_TYPES.includes(nodeType as MARKS);
+
+const isNodeTypeMark = (node: Node | Text | Mark): node is Mark => {
+  return isMarkType((<Mark>node).type as MARKS);
+};
+
+const isNodeTypeText = (node: Node | Text | Mark): node is Text => {
+  if (isNodeTypeMark(node)) {
+    return false;
+  }
+  if (node.nodeType === "text") {
+    return true;
+  }
+  return false;
+};
+
+const HTML_TAG_NODE_TYPES: Partial<
+  Record<HTMLTagName, BLOCKS | MARKS | INLINES>
+> = {
+  h1: BLOCKS.HEADING_1,
+  h2: BLOCKS.HEADING_2,
+  h3: BLOCKS.HEADING_3,
+  h4: BLOCKS.HEADING_4,
+  h5: BLOCKS.HEADING_5,
+  h6: BLOCKS.HEADING_6,
+  hr: BLOCKS.HR,
+  li: BLOCKS.LIST_ITEM,
+  ol: BLOCKS.OL_LIST,
+  p: BLOCKS.PARAGRAPH,
+  blockquote: BLOCKS.QUOTE,
+  table: BLOCKS.TABLE,
+  td: BLOCKS.TABLE_CELL,
+  th: BLOCKS.TABLE_HEADER_CELL,
+  tr: BLOCKS.TABLE_ROW,
+  ul: BLOCKS.UL_LIST,
+  b: MARKS.BOLD,
+  strong: MARKS.BOLD,
+  pre: MARKS.CODE,
+  i: MARKS.ITALIC,
+  sub: MARKS.SUBSCRIPT,
+  sup: MARKS.SUPERSCRIPT,
+  u: MARKS.UNDERLINE,
+  a: INLINES.HYPERLINK,
+  img: BLOCKS.EMBEDDED_ASSET,
+  video: BLOCKS.EMBEDDED_ASSET,
+  audio: BLOCKS.EMBEDDED_ASSET,
+};
+
+type Next = (node: HTMLNode) => Array<Node | Text | Mark>;
+
+const convertBlockNode =
+  (nodeType: BLOCKS) =>
+  (node: HTMLElementNode, next: Next): Array<Node | Mark | Text> => {
+    const block: Block = {
+      nodeType,
+      content: next(node) as Array<Block | Inline | Text>,
+      data: {},
+    };
+    return [block];
+  };
+
+const convertInlineNode =
+  (nodeType: INLINES) =>
+  (node: HTMLElementNode, next: Next): Array<Node | Mark | Text> => {
+    const inline: Inline = {
+      nodeType,
+      content: next(node) as Array<Inline | Text>,
+      data: {},
+    };
+    return [inline];
+  };
+
+const convertAnchorNode = (node: HTMLElementNode, next: Next) => {
+  const anchor: Inline = {
+    nodeType: INLINES.HYPERLINK,
+    content: next(node) as Array<Inline | Text>,
+    data: {
+      uri: node.attrs.href,
+    },
+  };
+  return [anchor];
+};
+
+const convertMarkNode = (nodeType: MARKS) => (_: HTMLElementNode, __: Next) => {
+  const mark: Mark = {
+    type: nodeType,
+  };
+  return [mark];
+};
+
+type ConvertNode = Record<
+  HTMLTagName,
+  (node: HTMLElementNode, next: Next) => Array<Node | Text | Mark>
+>;
+
+type ConvertMark = Record<
+  HTMLTagName,
+  (node: HTMLElementNode, next: Next) => Array<Node | Text | Mark>
+>;
+
+interface ConvertOptions {
+  convertNode: Partial<ConvertNode>;
+  convertMark: Partial<ConvertMark>;
+}
+
+const defaultConvertOptions: ConvertOptions = {
+  convertNode: {
+    h1: convertBlockNode(BLOCKS.HEADING_1),
+    h2: convertBlockNode(BLOCKS.HEADING_2),
+    h3: convertBlockNode(BLOCKS.HEADING_3),
+    h4: convertBlockNode(BLOCKS.HEADING_4),
+    h5: convertBlockNode(BLOCKS.HEADING_5),
+    h6: convertBlockNode(BLOCKS.HEADING_6),
+    hr: convertBlockNode(BLOCKS.HR),
+    li: convertBlockNode(BLOCKS.LIST_ITEM),
+    ol: convertBlockNode(BLOCKS.OL_LIST),
+    p: convertBlockNode(BLOCKS.PARAGRAPH),
+    blockquote: convertBlockNode(BLOCKS.QUOTE),
+    table: convertBlockNode(BLOCKS.TABLE),
+    td: convertBlockNode(BLOCKS.TABLE_CELL),
+    th: convertBlockNode(BLOCKS.TABLE_HEADER_CELL),
+    tr: convertBlockNode(BLOCKS.TABLE_ROW),
+    ul: convertBlockNode(BLOCKS.UL_LIST),
+    a: convertAnchorNode,
+    img: convertBlockNode(BLOCKS.EMBEDDED_ASSET),
+    video: convertBlockNode(BLOCKS.EMBEDDED_ASSET),
+    audio: convertBlockNode(BLOCKS.EMBEDDED_ASSET),
+  },
+  convertMark: {
+    b: convertMarkNode(MARKS.BOLD),
+    strong: convertMarkNode(MARKS.BOLD),
+    pre: convertMarkNode(MARKS.CODE),
+    i: convertMarkNode(MARKS.ITALIC),
+    sub: convertMarkNode(MARKS.SUBSCRIPT),
+    sup: convertMarkNode(MARKS.SUPERSCRIPT),
+    u: convertMarkNode(MARKS.UNDERLINE),
+  },
+};
+
+const createDocumentNode = (
+  content: TopLevelBlock[],
+  data: NodeData = {}
+): Document => ({
+  nodeType: BLOCKS.DOCUMENT,
+  data,
+  content,
+});
+
+const mapHtmlNodeToRichTextNode: Next = (node) => {
+  if (node.type === "text") {
+    const textNode: Text = {
+      nodeType: "text",
+      marks: [],
+      value: node.value,
+      data: {},
+    };
+    return [textNode];
+  }
+
+  const next: Next = (node) => {
+    if (node.type === "element") {
+      return node.children.flatMap((child) => mapHtmlNodeToRichTextNode(child));
+    }
+    return mapHtmlNodeToRichTextNode(node);
+  };
+
+  const nodeType = HTML_TAG_NODE_TYPES[node.tagName];
+
+  // Skip element if no node type is found
+  if (!nodeType) {
+    return next(node);
+  }
+
+  if (isBlockType(nodeType)) {
+    const converter = defaultConvertOptions.convertNode[node.tagName] ?? next;
+    const block = converter(node, next);
+    return block;
+  }
+
+  if (isInlineType(nodeType)) {
+    const converter = defaultConvertOptions.convertNode[node.tagName] ?? next;
+    const inline = converter(node, next);
+    return inline;
+  }
+
+  if (isMarkType(nodeType)) {
+    const converter = defaultConvertOptions.convertMark[node.tagName] ?? next;
+    const mark = converter(node, next);
+    const children = next(node);
+    return children.map((child) => {
+      if (isNodeTypeText(child)) {
+        child.marks = [...child.marks, ...mark.filter(isNodeTypeMark)];
+      }
+      return child;
+    });
+  }
+
+  throw new Error(`Unknown nodeType ${nodeType}`);
+};
+
+export const htmlStringToDocument = (htmlString: string): Document => {
+  const parsedHtml = parseHtml(htmlString);
+  const richTextNodes = parsedHtml.flatMap((node) =>
+    mapHtmlNodeToRichTextNode(node)
+  );
+  return createDocumentNode(richTextNodes as TopLevelBlock[]);
+};
diff --git a/src/parseHtml.ts b/src/parseHtml.ts
new file mode 100644
index 0000000..f2c8028
--- /dev/null
+++ b/src/parseHtml.ts
@@ -0,0 +1,64 @@
+import { parseFragment } from "parse5";
+import {
+  ChildNode,
+  Template,
+  DocumentType,
+  TextNode,
+  CommentNode,
+  Element,
+} from "parse5/dist/tree-adapters/default";
+import { isNotNull } from "./utils";
+
+import { HTMLNode, HTMLTagName } from "./types";
+
+const isChildNodeComment = (childNode: ChildNode): childNode is CommentNode => {
+  return childNode.nodeName === "#comment";
+};
+
+const isChildNodeTextNode = (childNode: ChildNode): childNode is TextNode => {
+  return childNode.nodeName === "#text";
+};
+
+const isChildNodeTemplate = (childNode: ChildNode): childNode is Template => {
+  return childNode.nodeName === "template";
+};
+
+const isChildNodeDocumentType = (
+  childNode: ChildNode
+): childNode is DocumentType => {
+  return childNode.nodeName === "#documentType";
+};
+
+const mapChildNodeToHtmlNode = (childNode: ChildNode): HTMLNode | null => {
+  if (
+    isChildNodeComment(childNode) ||
+    isChildNodeDocumentType(childNode) ||
+    isChildNodeTemplate(childNode)
+  ) {
+    return null;
+  }
+  if (isChildNodeTextNode(childNode)) {
+    return {
+      type: "text",
+      value: childNode.value,
+    };
+  }
+
+  return {
+    type: "element",
+    tagName: childNode.tagName as HTMLTagName,
+    children: childNode.childNodes
+      .map((c) => mapChildNodeToHtmlNode(c))
+      .filter(isNotNull),
+    attrs: Object.fromEntries(
+      childNode.attrs.map((attr) => [attr.name, attr.value])
+    ),
+  };
+};
+
+export const parseHtml = (htmlString: string): HTMLNode[] => {
+  const parsedHtml = parseFragment(htmlString);
+  return parsedHtml.childNodes
+    .map((node) => mapChildNodeToHtmlNode(node))
+    .filter(isNotNull);
+};
diff --git a/src/test/example.ts b/src/test/example.ts
new file mode 100644
index 0000000..085fbcd
--- /dev/null
+++ b/src/test/example.ts
@@ -0,0 +1,665 @@
+import { Document } from "@contentful/rich-text-types";
+
+export const EXAMPLE_RICH_TEXT = {
+  nodeType: "document",
+  data: {},
+  content: [
+    {
+      nodeType: "heading-2",
+      data: {},
+      content: [
+        {
+          nodeType: "text",
+          value: "Heading 2",
+          marks: [],
+          data: {},
+        },
+      ],
+    },
+    {
+      nodeType: "heading-3",
+      data: {},
+      content: [
+        {
+          nodeType: "text",
+          value: "Heading 3",
+          marks: [],
+          data: {},
+        },
+      ],
+    },
+    {
+      nodeType: "heading-4",
+      data: {},
+      content: [
+        {
+          nodeType: "text",
+          value: "Heading 4",
+          marks: [],
+          data: {},
+        },
+      ],
+    },
+    {
+      nodeType: "heading-5",
+      data: {},
+      content: [
+        {
+          nodeType: "text",
+          value: "Heading 5",
+          marks: [],
+          data: {},
+        },
+      ],
+    },
+    {
+      nodeType: "paragraph",
+      data: {},
+      content: [
+        {
+          nodeType: "text",
+          value: "Normal Text",
+          marks: [],
+          data: {},
+        },
+      ],
+    },
+    {
+      nodeType: "paragraph",
+      data: {},
+      content: [
+        {
+          nodeType: "text",
+          value: "Bold Text",
+          marks: [
+            {
+              type: "bold",
+            },
+          ],
+          data: {},
+        },
+      ],
+    },
+    {
+      nodeType: "paragraph",
+      data: {},
+      content: [
+        {
+          nodeType: "text",
+          value: "Italic Text",
+          marks: [
+            {
+              type: "italic",
+            },
+          ],
+          data: {},
+        },
+      ],
+    },
+    {
+      nodeType: "paragraph",
+      data: {},
+      content: [
+        {
+          nodeType: "text",
+          value: "Underlined Text",
+          marks: [
+            {
+              type: "underline",
+            },
+          ],
+          data: {},
+        },
+      ],
+    },
+    {
+      nodeType: "paragraph",
+      data: {},
+      content: [
+        {
+          nodeType: "text",
+          value: "Bold, Italic, and Underlined Sentence",
+          marks: [
+            {
+              type: "underline",
+            },
+            {
+              type: "italic",
+            },
+            {
+              type: "bold",
+            },
+          ],
+          data: {},
+        },
+      ],
+    },
+    {
+      nodeType: "paragraph",
+      data: {},
+      content: [
+        {
+          nodeType: "text",
+          value: "Mixed ",
+          marks: [],
+          data: {},
+        },
+        {
+          nodeType: "text",
+          value: "Bold",
+          marks: [
+            {
+              type: "bold",
+            },
+          ],
+          data: {},
+        },
+        {
+          nodeType: "text",
+          value: ", ",
+          marks: [],
+          data: {},
+        },
+        {
+          nodeType: "text",
+          value: "Italic",
+          marks: [
+            {
+              type: "italic",
+            },
+          ],
+          data: {},
+        },
+        {
+          nodeType: "text",
+          value: ", and ",
+          marks: [],
+          data: {},
+        },
+        {
+          nodeType: "text",
+          value: "Underlined",
+          marks: [
+            {
+              type: "underline",
+            },
+          ],
+          data: {},
+        },
+        {
+          nodeType: "text",
+          value: " Sentence",
+          marks: [],
+          data: {},
+        },
+      ],
+    },
+    {
+      nodeType: "hr",
+      data: {},
+      content: [],
+    },
+    {
+      nodeType: "paragraph",
+      data: {},
+      content: [
+        {
+          nodeType: "text",
+          value: "",
+          marks: [],
+          data: {},
+        },
+        {
+          nodeType: "hyperlink",
+          data: {
+            uri: "https://example.com",
+          },
+          content: [
+            {
+              nodeType: "text",
+              value: "Hyperlink",
+              marks: [],
+              data: {},
+            },
+          ],
+        },
+        {
+          nodeType: "text",
+          value: " to a URL",
+          marks: [],
+          data: {},
+        },
+      ],
+    },
+    {
+      nodeType: "paragraph",
+      data: {},
+      content: [
+        {
+          nodeType: "text",
+          value: "Sentence that is partially ",
+          marks: [],
+          data: {},
+        },
+        {
+          nodeType: "text",
+          value: "bold",
+          marks: [
+            {
+              type: "bold",
+            },
+          ],
+          data: {},
+        },
+        {
+          nodeType: "text",
+          value: " and partially a ",
+          marks: [],
+          data: {},
+        },
+        {
+          nodeType: "hyperlink",
+          data: {
+            uri: "https://example.com",
+          },
+          content: [
+            {
+              nodeType: "text",
+              value: "hyperlink",
+              marks: [],
+              data: {},
+            },
+          ],
+        },
+        {
+          nodeType: "text",
+          value: "",
+          marks: [],
+          data: {},
+        },
+      ],
+    },
+    {
+      nodeType: "paragraph",
+      data: {},
+      content: [
+        {
+          nodeType: "text",
+          value: "Sentence th",
+          marks: [],
+          data: {},
+        },
+        {
+          nodeType: "text",
+          value: "at is partially ",
+          marks: [
+            {
+              type: "bold",
+            },
+          ],
+          data: {},
+        },
+        {
+          nodeType: "hyperlink",
+          data: {
+            uri: "https://example.com",
+          },
+          content: [
+            {
+              nodeType: "text",
+              value: "overl",
+              marks: [
+                {
+                  type: "bold",
+                },
+              ],
+              data: {},
+            },
+            {
+              nodeType: "text",
+              value: "apping bold and",
+              marks: [],
+              data: {},
+            },
+          ],
+        },
+        {
+          nodeType: "text",
+          value: " hyperlink",
+          marks: [],
+          data: {},
+        },
+      ],
+    },
+    {
+      nodeType: "unordered-list",
+      data: {},
+      content: [
+        {
+          nodeType: "list-item",
+          data: {},
+          content: [
+            {
+              nodeType: "paragraph",
+              data: {},
+              content: [
+                {
+                  nodeType: "text",
+                  value: "Unordered List Item 1",
+                  marks: [],
+                  data: {},
+                },
+              ],
+            },
+          ],
+        },
+        {
+          nodeType: "list-item",
+          data: {},
+          content: [
+            {
+              nodeType: "paragraph",
+              data: {},
+              content: [
+                {
+                  nodeType: "text",
+                  value: "Unordered List Item 2",
+                  marks: [],
+                  data: {},
+                },
+              ],
+            },
+          ],
+        },
+      ],
+    },
+    {
+      nodeType: "ordered-list",
+      data: {},
+      content: [
+        {
+          nodeType: "list-item",
+          data: {},
+          content: [
+            {
+              nodeType: "paragraph",
+              data: {},
+              content: [
+                {
+                  nodeType: "text",
+                  value: "Ordered List Item 1",
+                  marks: [],
+                  data: {},
+                },
+              ],
+            },
+          ],
+        },
+        {
+          nodeType: "list-item",
+          data: {},
+          content: [
+            {
+              nodeType: "paragraph",
+              data: {},
+              content: [
+                {
+                  nodeType: "text",
+                  value: "Ordered List Item 2",
+                  marks: [],
+                  data: {},
+                },
+              ],
+            },
+          ],
+        },
+      ],
+    },
+    {
+      nodeType: "blockquote",
+      data: {},
+      content: [
+        {
+          nodeType: "paragraph",
+          data: {},
+          content: [
+            {
+              nodeType: "text",
+              value: "Blockquote",
+              marks: [],
+              data: {},
+            },
+          ],
+        },
+      ],
+    },
+    {
+      nodeType: "paragraph",
+      data: {},
+      content: [
+        {
+          nodeType: "text",
+          value: "",
+          marks: [],
+          data: {},
+        },
+      ],
+    },
+    {
+      nodeType: "table",
+      data: {},
+      content: [
+        {
+          nodeType: "table-row",
+          data: {},
+          content: [
+            {
+              nodeType: "table-header-cell",
+              data: {},
+              content: [
+                {
+                  nodeType: "paragraph",
+                  data: {},
+                  content: [
+                    {
+                      nodeType: "text",
+                      value: "Table Header Cell 1",
+                      marks: [],
+                      data: {},
+                    },
+                  ],
+                },
+              ],
+            },
+            {
+              nodeType: "table-header-cell",
+              data: {},
+              content: [
+                {
+                  nodeType: "paragraph",
+                  data: {},
+                  content: [
+                    {
+                      nodeType: "text",
+                      value: "Table Header Cell 2",
+                      marks: [],
+                      data: {},
+                    },
+                  ],
+                },
+              ],
+            },
+          ],
+        },
+        {
+          nodeType: "table-row",
+          data: {},
+          content: [
+            {
+              nodeType: "table-cell",
+              data: {},
+              content: [
+                {
+                  nodeType: "paragraph",
+                  data: {},
+                  content: [
+                    {
+                      nodeType: "text",
+                      value: "Table Cell AA",
+                      marks: [],
+                      data: {},
+                    },
+                  ],
+                },
+              ],
+            },
+            {
+              nodeType: "table-cell",
+              data: {},
+              content: [
+                {
+                  nodeType: "paragraph",
+                  data: {},
+                  content: [
+                    {
+                      nodeType: "text",
+                      value: "Table Cell BA",
+                      marks: [],
+                      data: {},
+                    },
+                  ],
+                },
+              ],
+            },
+          ],
+        },
+        {
+          nodeType: "table-row",
+          data: {},
+          content: [
+            {
+              nodeType: "table-cell",
+              data: {},
+              content: [
+                {
+                  nodeType: "paragraph",
+                  data: {},
+                  content: [
+                    {
+                      nodeType: "text",
+                      value: "Table Cell AB",
+                      marks: [],
+                      data: {},
+                    },
+                  ],
+                },
+              ],
+            },
+            {
+              nodeType: "table-cell",
+              data: {},
+              content: [
+                {
+                  nodeType: "paragraph",
+                  data: {},
+                  content: [
+                    {
+                      nodeType: "text",
+                      value: "Table Cell BB",
+                      marks: [],
+                      data: {},
+                    },
+                  ],
+                },
+              ],
+            },
+          ],
+        },
+        {
+          nodeType: "table-row",
+          data: {},
+          content: [
+            {
+              nodeType: "table-cell",
+              data: {},
+              content: [
+                {
+                  nodeType: "paragraph",
+                  data: {},
+                  content: [
+                    {
+                      nodeType: "text",
+                      value: "Table Cell ",
+                      marks: [],
+                      data: {},
+                    },
+                    {
+                      nodeType: "hyperlink",
+                      data: {
+                        uri: "https://example.com",
+                      },
+                      content: [
+                        {
+                          nodeType: "text",
+                          value: "Hyperlink",
+                          marks: [],
+                          data: {},
+                        },
+                      ],
+                    },
+                    {
+                      nodeType: "text",
+                      value: "",
+                      marks: [],
+                      data: {},
+                    },
+                  ],
+                },
+              ],
+            },
+            {
+              nodeType: "table-cell",
+              data: {},
+              content: [
+                {
+                  nodeType: "paragraph",
+                  data: {},
+                  content: [
+                    {
+                      nodeType: "text",
+                      value: "Table Cell Marks",
+                      marks: [
+                        {
+                          type: "bold",
+                        },
+                        {
+                          type: "italic",
+                        },
+                        {
+                          type: "underline",
+                        },
+                      ],
+                      data: {},
+                    },
+                  ],
+                },
+              ],
+            },
+          ],
+        },
+      ],
+    },
+    {
+      nodeType: "paragraph",
+      data: {},
+      content: [
+        {
+          nodeType: "text",
+          value: "",
+          marks: [],
+          data: {},
+        },
+      ],
+    },
+  ],
+} as Document;
diff --git a/src/test/index.test.ts b/src/test/index.test.ts
new file mode 100644
index 0000000..71737c8
--- /dev/null
+++ b/src/test/index.test.ts
@@ -0,0 +1,59 @@
+import { documentToHtmlString } from "@contentful/rich-text-html-renderer";
+import { BLOCKS, Document } from "@contentful/rich-text-types";
+import { htmlStringToDocument } from "../htmlStringToDocument";
+
+import { describe, expect, it } from "vitest";
+import { EXAMPLE_RICH_TEXT } from "./example";
+
+// https://www.contentful.com/developers/docs/tutorials/general/getting-started-with-rich-text-field-type/
+const richTextDocument: Document = {
+  nodeType: BLOCKS.DOCUMENT,
+  data: {},
+  content: [
+    {
+      nodeType: BLOCKS.PARAGRAPH,
+      content: [
+        {
+          nodeType: "text",
+          marks: [],
+          value: "I am an odd paragraph.",
+          data: {},
+        },
+      ],
+      data: {},
+    },
+    {
+      nodeType: BLOCKS.PARAGRAPH,
+      content: [
+        {
+          nodeType: "text",
+          marks: [],
+          value: "I am even.",
+          data: {},
+        },
+      ],
+      data: {},
+    },
+  ],
+};
+
+const htmlString = documentToHtmlString(EXAMPLE_RICH_TEXT);
+
+describe("Parse HTML string to Contentful Document", () => {
+  /*it("Parse string to generic HTML nodes", () => {
+    const htmlNodes = parseHtml(CISION_EXAMPLE);
+    expect(htmlNodes).toEqual([]);
+  });*/
+
+  /*it("Parse HTML string to Contentful Rich Text", () => {
+    const htmlNodes = htmlStringToDocument(CISION_EXAMPLE);
+    const newHtmlString = documentToHtmlString(htmlNodes);
+    expect(newHtmlString).toEqual(CISION_EXAMPLE);
+  });*/
+
+  it("Parse HTML string to Contentful Rich Text", () => {
+    const htmlNodes = htmlStringToDocument(htmlString);
+    const newHtmlString = documentToHtmlString(htmlNodes);
+    expect(newHtmlString).toEqual(htmlString);
+  });
+});
diff --git a/src/types.ts b/src/types.ts
new file mode 100644
index 0000000..62daa65
--- /dev/null
+++ b/src/types.ts
@@ -0,0 +1,15 @@
+export type HTMLTagName = keyof HTMLElementTagNameMap;
+
+export interface HTMLTextNode {
+  type: "text";
+  value: string;
+}
+
+export interface HTMLElementNode {
+  type: "element";
+  tagName: HTMLTagName;
+  children: HTMLNode[];
+  attrs: { [attr: string]: string };
+}
+
+export type HTMLNode = HTMLElementNode | HTMLTextNode;
diff --git a/src/utils.ts b/src/utils.ts
new file mode 100644
index 0000000..202f20c
--- /dev/null
+++ b/src/utils.ts
@@ -0,0 +1,2 @@
+export const isNotNull = <T>(value: T): value is Exclude<T, null> =>
+  value !== null;