Integrate `decoding="async"` logic into `wp_get_loading_optimization_attributes()`

[flixos90]

The wp_get_loading_optimization_attributes() function was added in WordPress 6.3 as a central way to apply loading attributes to specific elements/tags. One of the attributes that WordPress core has supported since 6.1 (see #53232) is decoding="async". This attribute is currently still manually applied, instead it should be applied as part of the new function going forward.

This ticket's implementation should be considered blocked by #58891 as making the changes before that would be unnecessarily complicated and error-prone.

[flixos90]

With [56347] committed, this effort is now unblocked. Please refer to the latest codebase for any changes to wp_get_loading_optimization_attributes().

## What's in:

• [x] Moved the decoding attribute of theimg tag into wp_get_loading_optimization_attributes();
• [x] Deprecation of wp_img_tag_add_decoding_attr() and it's filters;
• [x] Add and update tests relevant to the ticket

Trac ticket: 58892

Have you get decoding attribute in content images?

[mukesh27]

Thanks @pereirinha for the PR. It's a great beginning. I've provided some feedback that you might want to take into account.

@mukeshpanchal27 @joemcgill

Thanks for your comments. I'm actively working on them and pushing updates frequently.
I'll comment when I'm done so you can circle back on this.

Thank you all for the feedback. I'll be working on it.

Thanks, @mukeshpanchal27 @joemcgill @felixarntz, for all the comments so far.

I left you some comments back, where it made sense, and updated the code. There are still some pieces I'm trying to wrap my head around, and a quick call would be very helpful to clear them.

If it makes sense, let's talk on Slack and try to conciliate a time for it.

[flixos90]

The 4 tickets related to wp_get_loading_optimization_attributes() should receive a single dev note post explaining those changes.

[flixos90]

In 56690:

Media: Rely on wp_get_loading_optimization_attributes() to add decoding="async" to images.

The wp_get_loading_optimization_attributes() function was introduced in 6.3, as a single centralized place to control loading optimization attributes for various tags, most importantly images.

This changeset consolidates the decoding="async" optimization, which was added in 6.1, to occur solely as part of wp_get_loading_optimization_attributes(), removing duplicate code and allowing centralized filtering based on [56651].

As part of the change, the wp_img_tag_add_decoding_attr() function has been deprecated. The filter of the same name continues to be maintained for backward compatibility, as before covering only images that are part of a content blob such as post content (the_content).

Props pereirinha, mukesh27, joemcgill, flixos90.
Fixes #58892.
See #53232.

Committed in https://core.trac.wordpress.org/changeset/56690

[spacedmonkey]

Reopenning, as this commit resulted in a notice error. See attached screenshot.

This PR fixes a regression introduced on #58892 when ​moving the responsibility for the decoding attribute to the wp_get_loading_optimization_attributes.

This will add the decoding attribute to the get_avatar(). As trunk currently is, this context will fail to get any optimization attributes while running the_content filter.

Trac ticket: #58892

This PR fixes a regression introduced on #58892 when ​moving the responsibility for the decoding attribute to the wp_get_loading_optimization_attributes.

This will add the decoding attribute to the get_avatar(). As trunk currently is, this context will fail to get any optimization attributes while running the_content filter.

Trac ticket: #58892

[pereirinha]

This Slack thread holds more information about the reopening and discussion around the source of the issue and mitigatory measures.

In sum, there's a regression introduced here ​https://github.com/WordPress/wordpress-develop/pull/4991/commits/828130034c5a7790ed8e53660b34eeaf214b4e0d#diff-2dd87d67e80339[…]3df894da4462e69L2848/, where the default value for decoding was removed.
As you see, I also removed it in various other places, and I wasn't able to get any issues. The reason for that is that we get the decoding attribute via the wp_get_loading_optimization_attributes() that we ran after.

The problem with the gravatar is that it is requested within the context of get_avatar, but when we are doing the the_content filter. This will fail this ​check so we end up without the decoding attribute.

The recommendation is to bring the decoding attribute as a default value as it was for the get_avatar(), but I don't think we need to bring the other ones.

An alternative approach suggested by @spacedmonkey is to move the setting of the decoding attribute before the context checks, as this argument has a different type of impact than lazy loading and fetchpriority.

@felixarntz @mukeshpanchal27

I made the change and didn't find any issues.

[flixos90]

In 56756:

Media: Prevent PHP notice in get_avatar().

Follow up fix to [56690].

Props pereirinha, mukesh27, spacedmonkey.
Fixes #58892.

Committed in https://core.trac.wordpress.org/changeset/56756

[flixos90]

Dev note published: ​https://make.wordpress.org/core/2023/10/18/image-loading-optimization-enhancements-in-6-4/