Official statement
The async attribute tells the browser that it can download other resources in the background and execute the script as soon as it's ready. The defer attribute is more polite: it downloads in the background but does not interrupt rendering and executes after the complete parsing of the page.
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Official statement from
(5 years ago)
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Are third-party scripts really sabotaging your Core Web Vitals even when they're...
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TL;DR
Using the async attribute allows resources to be downloaded without interrupting the user experience. The defer attribute ensures your scripts run only after the page parsing is complete. This is crucial for fast rendering and improved SEO efficiency.
❓ Frequently Asked Questions
Async et defer sont-ils interchangeables ?
Non, ils s'appliquent à des contextes différents. Async télécharge et exécute immédiatement, defer attend la complétion du parsing.
Peut-on utiliser async et defer simultanément ?
Non, choisir l'un ou l'autre selon le comportement de rendu désiré est préférable.
Quels outils utiliser pour vérifier l'impact ?
PageSpeed Insights et Lighthouse sont efficaces pour analyser l'effet de ces optimisations.
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