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What does "rendering optimization" mean?

Frontend ToolsJunior~1 min read
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Rendering optimization is the process of speeding up and simplifying how the browser displays a page on the screen.

What happens after the HTML loads

To understand what this means, you need to know what happens after the HTML loads:

  1. HTML is parsed → the DOM is built
  2. CSS is parsed → the CSSOM is created
  3. DOM + CSSOM are combined → render tree
  4. Layout (reflow) - positions and sizes are calculated
  5. Paint - elements are drawn
  6. Composite - everything is assembled into the final frame on screen

What rendering optimization means

  • reduce the number and complexity of these steps
  • avoid unnecessary recalculations (layout thrashing)
  • avoid forcing the browser to repaint everything without reason
  • reduce the load on the main thread

Examples of what slows down rendering

  • frequent DOM operations or inline styles via JavaScript
  • animations that do not run through transform or opacity
  • large images without dimensions set in CSS
  • complex CSS selectors and deeply nested layout
  • synchronous JS scripts in <head>

Goal of optimization

Make the page render fast, smoothly, without jank or "white screens", especially on low-power devices.

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