Why can JavaScript slow initial rendering?
Synchronous scripts block HTML parsing and can delay DOM construction, which delays the rest of the rendering pipeline.
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More FAQs in What Happens in the Browser Before Rendering?
It parses HTML into the DOM, parses CSS into the CSSOM, runs scripts, computes layout, and then paints the page.
The browser needs CSS to know how elements should be styled before it can safely paint the page.
Layout calculates element size and position. Paint draws the visual output after layout is known.
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