How Browser Rendering Works and How to Eliminate CSS/JS Blocking
This article explains the browser's rendering pipeline—how HTML and CSS are parsed into DOM and CSSOM trees, how JavaScript can block rendering, and practical techniques like inlining critical CSS and moving scripts to the page bottom to improve load performance.
Browser Rendering Steps
(1) The browser parses HTML to build the DOM tree (Document Object Model). (2) It parses CSS to build the CSSOM tree (CSS Object Model). (3) Before combining DOM and CSSOM into the render tree, JavaScript files are parsed and executed.
What Blocks the Render Tree?
(1) Render‑blocking CSS
When constructing the CSSOM, all CSS files are downloaded regardless of whether they are used on the current page. To reduce blocking, inline the critical CSS needed for the initial view into the <head> and remove unused CSS.
How to find unused CSS? Use PageSpeed Insights to get statistics on unused CSS and render‑blocking resources, then run tools such as gulp-uncss or grunt-uncss to eliminate them.
(2) Render‑blocking JavaScript
If the parser encounters a <script> tag while parsing HTML, parsing pauses until the script is fetched and executed. Therefore, place scripts at the bottom of the page and remove non‑essential JavaScript references.
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