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Idiosyncrasies of the HTML parser

This book is 50% completeLast updated on 2026-06-12
Simon Pieters

By understanding how the HTML parser works, you will become more confident in writing correct HTML, understand how it affects things for scripting and styling, and recognize how to address some security issues. Additionally, this book may inspire you to implement your own HTML parser.

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About

About

About the Book

The HTML parser is a piece of software that processes HTML markup and produces an in-memory tree representation (known as the DOM).

The HTML parser has many strange behaviors. This book will highlight the ins and outs of the HTML parser, and contains almost-impossible quizzes.

HTML is not only used by basically all of the web, but it is also part of many modern applications. The HTML parser is part of the foundation of the web platform.

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License

Creative Commons Attribution 4.0 International License

Author

About the Author

Simon Pieters

Simon works on web standards and testing the web platform to foster interoperability between web browsers. He is one of the editors of the WHATWG HTML Living Standard, and has helped the research and design of the HTML parser specification.

Contents

Table of Contents

Preface

  1. Intended audience
  2. Definition
  3. Scope
  4. Practical application
  5. About the author
  6. Acknowledgements
  7. Contribute

Chapter 1. Introduction

  1. The DOM, parsing, and serialization
  2. History of HTML parsers
  3. The HTML parser is specified

Chapter 2. The HTML syntax

  1. The doctype
  2. Elements
  3. Documents
  4. Start tags
  5. End tags
  6. Attributes
  7. Optional tags
  8. Character references
  9. CDATA sections
  10. Comments

Chapter 3. The HTML parser

  1. Overview of the HTML parser
  2. Error handling
  3. Detecting character encoding
  4. Preprocessing the input stream
  5. Tokenizer
  6. Tree construction
  7. Tags that are no longer supported

Chapter 4. Scripting complications

  1. Revised overview of the HTML parser
  2. document.write()
  3. Other parser APIs
  4. DOM manipulation

Chapter 5. Serializing

Chapter 6. Security implications

  1. Introduction
  2. Case studies
  3. Best practice

Appendix A. Implementations

Appendix B. Conformance checkers

  1. DTD-based validators
  2. Validator.nu

Appendix C. Microsyntaxes

  1. Numbers
  2. Image map coordinates
  3. Responsive images
  4. Colors
  5. Meta refresh

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