Consider adding "defer" attribute to <script src=> tags

[jeff-at-livecanvas]

Consider adding "defer" attribute to <script> statements that recall JS files.
It could help for pageSpeed and make sense as those scripts are generally for navigational elements that user will interact with after page shows.

[eliot-akira]

Thank you for the feature suggestion.

Support for script defer attribute was added in WP 6.3.

• Registering scripts with async and defer attributes in WordPress 6.3

wp_register_script( 
  'foo',
  '/path/to/foo.js',
  [],
  '1.0.0',
  [
    'in_footer' => true,
    'strategy' => 'defer'
  ]
);

For prior/existing usage of the wp_register_script() and wp_enqueue_script() functions making use of the $in_footer boolean param, backward compatibility is retained via logic that explicitly sets the scripts group to the applicable value for footer or header printing based on the boolean value passed to the new/overloaded $args parameter. Thus, full backward compatibility is retained and this is a non-breaking enhancement of the API.

---

The async attribute cannot be used for our purposes, since it can change the order of script execution.

In addition, apparently there's some complexity in how scripts with attached inline script (usually used for passing data from PHP to JS) are loaded.

if a given script handle contains inline scripts in the after position, this script will be assumed to be blocking and any intended strategy such as defer/async will be removed, with the final eligible strategy being blocking. This, in turn, may affect the script’s dependency tree, and all scripts within it, may too, be treated as blocking scripts in a bid to retain correct execution order and functionality of the enqueued scripts. Logic is explicitly employed to ensure correct execution order of the script tree in these instances.

I'll need to check exactly how this works when the defer attribute is added to the main script.