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Clarify Goals:
- Parse HTML into a structured representation (like a tree or graph).
- Capture HTML tags, their attributes, and their content (text or child elements).
- Allow navigation and extraction of elements, attributes, or content.
Determine Extensibility Needs:
- Should it support HTML5-specific quirks and malformed HTML?
- Should it handle nested structures like <div><p>Text</p></div>?
- Should it be tolerant of invalid or missing tags?
Output Requirements:
- Decide the data structure (e.g., a tree with nodes representing elements).
- Define an API for navigation and queries.

Parsing Mode:
- Top-Down Recursive Descent Parsing: Useful for simplicity and clarity.
- Lexer + Parser Combo: Use a tokenizer to break the input into tokens, then parse the tokens into a tree.
- Stream-Based Parsing: If you need to process large HTML documents efficiently.
Tokenizer Logic:
- Split the HTML into meaningful tokens (e.g., tags, attributes, content).
- Handle special cases such as:
  - Self-closing tags (<img />).
  - Comment tags ().
  - Void elements (<br>, <meta>, etc.).
Error Handling:
- Decide how to handle malformed HTML (e.g., unclosed tags, incorrect nesting).
- You can choose to tolerate these errors or strictly validate the input.

Tree Representation:
- Each node represents an HTML element.
- Each node stores:
  - Tag name (e.g., div, p).
  - Attributes (as key-value pairs, e.g., class="foo").
  - Child nodes (for nested elements).
  - Inner text (for leaf nodes or mixed content).
Example (Go Pseudo-Struct):
```
type Node struct {
    Tag       string
    Attributes map[string]string
    Children  []*Node
    Text      string
}
```
Navigation Features:
- Provide methods to traverse:
  - Depth-first (DOM-style tree traversal).
  - Breadth-first or custom filtering (e.g., find by tag, attribute, or value).

Lexical Analysis:
- Tokenize the HTML document into:
  - Start tags (<div>), end tags (</div>), attributes (key="value"), and text.
Syntactic Parsing:
- Construct the tree structure by:
  - Starting new nodes when encountering a start tag.
  - Closing nodes when encountering end tags.
  - Adding text content to the current node.
Error Recovery:
- Handle situations like missing end tags or stray characters gracefully.

Navigation:
- Traverse the tree with methods like GetElementByTag("tag"), GetElementByID("id"), etc.
- Support advanced queries (e.g., CSS selector-like matching).
Manipulation:
- Allow updating attributes or text content in the tree.
- Provide a way to serialize the tree back into valid HTML.
Extraction:
- Enable easy extraction of:
  - Specific tags (e.g., all <a> tags with href attributes).
  - Attribute values (e.g., src of <img> tags).
  - Enclosed text.

Performance Optimization:
- Optimize the tokenizer for speed (e.g., regex vs. manual parsing).
- Efficient tree traversal (cache lookups or indexes for frequently queried nodes).
Extensibility:
- Allow adding custom handlers for specific tags or attributes.
- Make it modular to handle future needs (e.g., extending for XML or XHTML).

Test Cases:
- Validate with simple HTML, deeply nested HTML, malformed HTML, and large documents.
- Include edge cases like self-closing tags, void elements, and attributes without values.
Real-World Scenarios:
- Parse and navigate HTML from popular websites or real-world examples to ensure robustness.

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