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 <figure> and <figcaption> 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]

Maybe the prefix should have - added to the end, so that it doesn't result in the attachment ID having an extra digit added to the end of it. Then to address the issue of back-compat for existing selectors, it could simply replace -1 from the end of the string. This would allow true duplicates to then start getting numbered with -2, -3,-4, and so on. This would match how attachment filenames are incremented to avoid duplicates.

Suggested change

	$unique_id_value = wp_unique_id( sanitize_html_class( $atts['id'] ) );
	$unique_id_value = preg_replace( '/-1$/', '', wp_unique_id( sanitize_html_class( $atts['id'] . '-' ) ) );

[westonruter]

Actually, this won't work. The counter is global.

So we'd need to basically need to implement our own wp_unique_id(), but keeping track of which prefixes have been used.

[westonruter]

oh, and that's a thing! wp_unique_prefixed_id().

Suggested change

	$unique_id_value = wp_unique_id( sanitize_html_class( $atts['id'] ) );
	$unique_id_value = preg_replace( '/-1$/', '', wp_unique_prefixed_id( sanitize_html_class( $atts['id'] . '-' ) ) );

[westonruter]

Looks like the unit tests need to be updated now.

[SteelWagstaff]

Thanks @westonruter -- I'm traveling up to Portland today but will try to get to this when I have a spare minute. Question about approach. Right now I've been leaving the duplicate if checks, but wonder whether it might be cleaner/simpler to consolidate the logic using a single ternary to determine the caption_id_source, then use another ternary for the final unique ID. This eliminates the redundant if blocks:

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

$caption_id_source = $atts['caption_id']
	? sanitize_html_class( $atts['caption_id'] )
	: ( $atts['id'] ? 'caption-' . str_replace( '_', '-', sanitize_html_class( $atts['id'] ) ) : '' );

if ( $caption_id_source ) {
	// Use the figure's unique ID to derive caption ID when caption_id came from id.
	$caption_id_value = ( ! empty( $unique_id_value ) && 0 === strpos( $caption_id_source, 'caption-' ) )
		? 'caption-' . $unique_id_value
		: wp_unique_id( $caption_id_source );
	$caption_id       = 'id="' . esc_attr( $caption_id_value ) . '" ';
	$describedby      = 'aria-describedby="' . esc_attr( $caption_id_value ) . '" ';
}

For me, this feels cleaner because a single line would determines what the caption_id source should be (explicit provided value, derived from id, or empty), it removes the need to modify $atts['caption_id'] first, it ensures that caption_id_source vs caption_id_value is obvious, and preserves the guarantee of uniqueness. Thoughts?

[westonruter]

Suggested change

	$caption_id_value = preg_replace( '/-1$/', '', wp_unique_id( $atts['caption_id'] . '-' ) );
	$caption_id_value = preg_replace( '/-1$/', '', wp_unique_prefixed_id( $atts['caption_id'] . '-' ) );

[westonruter]

It seems this suggestion wasn't applied.

[westonruter]

@SteelWagstaff That refactor seems like it could be a good idea, but let's first fix the underlying issue and get the jobs passing before we consider that refactor.

[SteelWagstaff]

@westonruter I've updated the functions and checked that all tests pass. Figure and figure caption IDs are now guaranteed to be unique.

[westonruter]

Suggested change

	$caption_id_value = wp_unique_id( $atts['caption_id'] );
	$caption_id_value = preg_replace( '/-1$/', '', wp_unique_prefixed_id( $atts['caption_id'] ) );

[westonruter]

@SteelWagstaff Did you see my suggestions regarding the use of wp_unique_prefixed_id() and stripping off the -1?

[SteelWagstaff]

@westonruter Yes, but I think I misunderstood its purpose. I just read the function description in https://developer.wordpress.org/reference/functions/wp_unique_prefixed_id/ and it now makes more sense to me. Like you, I'm trying to think of the best way to ensure unique IDs AND preserve backwards compatibility for existing CSS selectors. There have been so many suggestions going back and forth I wasn't sure where things stood.

Here's my understanding of what we want to try:

• Give each figure and figcaption a unique ID on their first usage on the page (e.g. a figure with id="attachment_6" and figcaption with id="caption_attachment_6").
• If the same image is inserted a second time, increment the unique ID for the figure and figcaption by adding -2, -3 as a suffix to each additional occurrence. This would mean, that its second usage would result in the figure being assigned an id of "attachment_6-2" and the figcaption would be "caption_attachment_ 6-2", and so on.

Does that sound right to you? If we agree on the psuedo-code, I can then implement this in the relevant code block.

[westonruter]

@SteelWagstaff yes, that sounds right. The problem with the existing wp_unique_id() is that it adds the incremented suffix irrespective of the prefix, and we want to strip off -1 from the first instance.