fix: ensure that figure and figcaption elements have unique IDs

[SteelWagstaff]

Ensure that figure and figcaption elements have unique IDs when inserted into a page.

Problem:
When the same image is inserted multiple times on a page, the

and elements receive duplicate HTML IDs. This violates HTML validity requirements (IDs must be unique) and breaks accessibility features like aria-describedby that depend on ID references

Trac ticket: https://core.trac.wordpress.org/ticket/65315#ticket

Demonstration of HTML changes

Before PR (notice duplicate figure ID and figcaption ID attributes for all three images:

After PR (notice unique figure ID and figcaption ID attributes for each image occurence.

Use of AI Tools

AI assistance: Yes
Tool(s): GitHub Copilot Pro (integrated in IDE), Skills provided by obra/superpowers
Model(s): Claude Haiku 4.5
Used for: Writing tests; code changes and comments written by me.

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Props steelwagstaff, westonruter, mukesh27.

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[github-actions]

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[westonruter]

When reviewing this, Claude pointed out something useful:

A thought on stability of the id for the single-caption case (non-blocking)

Just want to flag something for consideration — not a blocker, and the approach here makes sense for the problem it's solving.

In src/wp-includes/media.php:

if ( $atts['id'] ) {
    $unique_id_value = wp_unique_id( sanitize_html_class( $atts['id'] ) );
    $id              = 'id="' . esc_attr( $unique_id_value ) . '" ';
}

Since this runs every caption's id through wp_unique_id(), the suffix gets appended unconditionally — including the very common case of a single [caption id="attachment_123"] on a page, where the id was already unique. So the output shifts from:

<figure id="attachment_123">

to something like:

<figure id="attachment_1231">

…with the suffix depending on how many times wp_unique_id() was called earlier in the request.

The reason I mention it: the id is no longer the author-supplied value, so anything that referenced it by its literal id could be affected — e.g. in-content anchors (#attachment_123), theme/plugin CSS (#attachment_123 { … }), or JS (getElementById). Probably an edge case in practice, but worth being aware of since the id had historically been stable.

To be clear, this isn't an accessibility regression — the aria-describedby ↔ figcaption id linkage stays internally consistent (both come from the same $caption_id_value), so the association still resolves correctly.

If preserving backward compatibility for the common path feels worthwhile, one option would be to only disambiguate on an actual collision (suffix the 2nd+ occurrence, leave the first as-is). That's more logic than the current approach, though, and may not be worth it depending on how often duplicate captions actually occur — happy to defer to your judgment here.

It seems we should only add the unique suffix for when the ID is truly going to be a duplicate. This will preserve existing CSS which may be targeting an element by a given ID. This would preserve back-compat while also fixing the accessibility issue.

[westonruter]

Looks like the unit tests need to be updated now.