The defer attribute gives a hint to the browser that the script does not create any content so the browser can optionally defer interpreting the script. This can improve performance by delaying execution of scripts until after the body content is parsed and rendered.
https://www.websiteoptimization.com/speed/tweak/defer/
Document Standards
We've got no data for this technology yet sorry. It may have been added recently or exist here for historical purposes.
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