html-parsing

The browser receives HTML as a text string from the server. To build the DOM and render the page, the browser needs to convert this text into a structured tree. Raw HTML isn’t programmable.

HTML parsing tokenizes HTML text and constructs the DOM tree. The parser is lenient—it corrects malformed HTML automatically (missing closing tags, mismatched tags, etc.).

1. Tokenization: Break HTML into tokens (<div>, class=, text content, </div>)
2. Tree construction: Assemble tokens into DOM tree

   <html>
     <body>
       <h1>Hello</h1>
   

   ↓ DOM: html → body → h1 → “Hello”

3. Error recovery: Fix common HTML errors automatically

The HTML parser can be paused by <script> tags (unless async/defer).

• Lenient parsing: fixes errors automatically
• Scripts block parsing (unless async/defer)
• Incremental parsing: starts before full HTML arrives
• Output is the DOM tree

• Builds into: DOM Tree — HTML parsing creates the DOM
• Related: Browser Rendering — HTML parsing is step 1
• Related: CSS Parsing — parallel but separate process
• Contrasts with: XML Parsing — HTML is lenient, XML is strict

• <script> without async/defer blocks parsing
• document.write() during parsing can break things
• Malformed HTML gets “fixed” (may not match intent)
• Table parsing is especially complex