Search inputs: avoid unnecessary mismatch between visible label and accessible name

[afercia]

Description

Similar to https://github.com/WordPress/gutenberg/issues/65112

See the above issue for more context.

When possible, the visual labeling of controls should match the actual accessible name (provided for example by a visually hidden <label> element, or an aria-label or aria-labelledby attributes).

All the search inputs in the editor should be audited and any mismatch avoided.

As an example, the Search input in the Styles panel > Blocks is visually labeled via its placeholder attribute. This is already arguable, as placeholders should not be used as replacement for labels.

• The placeholder value is Search.
• The actual accessible name is Search for blocks.

While the added context may help blind screen reader users, it doesn't help other users and may even be an accessibility barrier, for example for sighted screen reader users and speech recognition / voice control users.

Step-by-step reproduction instructions

• Go to Site Editor > Styles > Blocks.
• Observe the search input placeholder is the only visible 'label' of the input and it's Search.
• Observe the actual accessible name provided by a visually hidden <label> element is Search for blocks.

Screenshots, screen recording, code snippet

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Environment info

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Please confirm that you have searched existing issues in the repo.

• [X] Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

• [X] Yes

[afercia]

There's plenty of cases of this kind of mismatch. For example, the main Inserter search input:

• Placeholder: Search.
• Actual label: Search for blocks and patterns. Notice that it is also inaccurate, as the Inserter now contains blocks, patterns, and media.

Screenshot revealing the visually hidden label:

Same for the link control, etc.: