HTML

what is defer in html

The defer attribute is a boolean attribute.

If the defer attribute is set, it specifies that the script is downloaded in parallel to parsing the page, and executed after the page has finished parsing.

Note: The defer attribute is only for external scripts (should only be used if the src attribute is present).

Note: There are several ways an external script can be executed:

If async is present: The script is downloaded in parallel to parsing the page, and executed as soon as it is available (before parsing completes)
If defer is present (and not async): The script is downloaded in parallel to parsing the page, and executed after the page has finished parsing
If neither async or defer is present: The script is downloaded and executed immediately, blocking parsing until the script is completed

Browser support
Chrome - 8.0
Edge - 10.0
Firefox - 3.5
Safari - 5.0
Opera - 15.0

defer in html

The defer attribute is a boolean attribute.

If the defer attribute is set, it specifies that the script is downloaded in parallel to parsing the page, and executed after the page has finished parsing.

Note: The defer attribute is only for external scripts (should only be used if the src attribute is present).

Note: There are several ways an external script can be executed:

If async is present: The script is downloaded in parallel to parsing the page, and executed as soon as it is available (before parsing completes)
If defer is present (and not async): The script is downloaded in parallel to parsing the page, and executed after the page has finished parsing
If neither async or defer is present: The script is downloaded and executed immediately, blocking parsing until the script is completed

Browser support
Chrome - 8.0
Edge - 10.0
Firefox - 3.5
Safari - 5.0
Opera - 15.0