Problem/Motivation
HTML5 proposes/solidifies asynchronous loading of JavaScript files based on two attributes: defer and async. Drupal has supported defer since at least D6, and support for async is currently in the works (#1140356: Add async, onload property to script tags).
Current behavior (as of 8.6) is that scripts with the defer attribute do not ever get aggregated (even in the case where there are multiple scripts added through one library).
Proposed resolution
We should have a list of attributes that support aggregation.
JS files with these attributes should be aggregated together if they have the same attribute values and we should make sure the attributes are on the bundle.
This means that if a library defines 3 JS files with the defer attribute, we should create a single bundle with the defer attribute.
Remaining tasks
- Related issue: #1140356: Add async, onload property to script tags
- Related issue: #865536: drupal_add_js() is missing the 'browsers' option
- Patch, test, etc.
User interface changes
None
API changes
Though this affects JavaScript aggregation, this should have little-to-no API impact.