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
defer attribute—blocks rendering, and pauses parsing of HTML.
- Parse on Wikipedia