AMP error due to invalid attribute fetchpriority

[pereirinha]

Bug Description

The fetchpriority feature is currently in experimental mode, and it's currently not supported by AMP.

This will cause content pieces to be flagged with an error related to an invalid attribute, and there might be a conflict with the motto Enhancement for most, regression for none.

Steps to reproduce

1. Ensure you have the Performal Lab and AMP plugins installed and active;
2. Go to the AMP Settings page and make sure to use either Standard or Transitional template mode;
3. Visit a published post/page in the FE that has an image;
4. In the admin bar, client the menu AMP, and click View AMP version. If this menu option isn't available, it might be that you're already seeing the AMP version;
5. You should see a yellow icon next to the AMP word. Expand the menu and click Validate URL;
6. You should see a similar error as the attached screenshot;

Screenshots

[swissspidy]

This should be reported at https://github.com/ampproject/amp-wp/

WordPress is adding fetchpriority correctly to <img> tags, but then the AMP plugin is turning those into <amp-img> and not removing the attribute.

FWIW you can use add_filter( 'amp_native_img_used', '__return_true' ); to keep using the native <img> even when using AMP — it's totally supported and valid AMP.

[felixarntz]

Agreed, fetchpriority is in fact no longer experimental but part of the HTML standard. If amp-img does not support it, I think it needs to be resolved on that side.

Given that AMP is not associated with the WordPress core project and not a web standard either, I don't think it would be appropriate to fix this in the Performance Lab plugin.

[westonruter]

This first has to be allowed by the AMP validator before it is allowed by the AMP plugin. For that, please see ampproject/amphtml#38715

[pereirinha]

Thanks all for taking the time to evaluate this one.