Parse

Parsing means analyzing and converting a program into an internal format that a runtime environment can actually run, for example the JavaScript engine inside browsers.

The browser parses HTML into a DOM tree. HTML parsing involves tokenization and tree construction. HTML tokens include start and end tags, as well as attribute names and values. If the document is well-formed, parsing it is straightforward and faster. The parser parses tokenized input into the document, building up the document tree.

When the HTML parser finds non-blocking resources, such as an image, the browser will request those resources and continue parsing. Parsing can continue when a CSS file is encountered, but <script> tags—particularly those without an async or defer attribute—blocks rendering, and pauses parsing of HTML.