Prevent loading=lazy from being copied from iframe to amp-iframe since invalid (and built-in)

[westonruter]

Bug Description

When adding an <iframe> with loading=lazy attribute, a validation error is raised because of this loading attribute. The AMP_Iframe_Sanitizer should be just omitting the loading attribute entirely when it has a value of lazy since amp-iframe does lazy-loading by default and so it is redundant.

If the value is anything other than lazy (e.g. eager or auto) then a validation error should still be raised so the user is informed of the failed attempt to lazy-load.

Relates to #3938.

Expected Behaviour

No validation error should be raised when loading=lazy is added to an <iframe>.

Steps to reproduce

1. Create a Custom HTML block with <iframe loading="lazy" src="https://example.com" width="320" height="640"></iframe>
2. Save the post.
3. See the validation error.

Screenshots

Additional context

• WordPress version:
• Plugin version:
• Gutenberg plugin version (if applicable):
• AMP plugin template mode:
• PHP version:
• OS:
• Browser: [e.g. chrome, safari]
• Device: [e.g. iPhone6]

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Acceptance criteria

Implementation brief

QA testing instructions

1. Create a Custom HTML block with <iframe loading="lazy" src="https://example.com" width="320" height="640"></iframe>

   1. Save the post.
   2. No validation errors should be reported.

2. Create a Custom HTML block with <iframe loading="eager" src="https://example.com" width="320" height="640"></iframe>

   1. Save the post.
   2. The following validation error should be reported:

Demo

Changelog entry

[claudiosossi]

Verified in QA