JuZ Rich Text Extender allows developers to register custom inline format buttons in the Gutenberg (block editor) toolbar. Each button applies a specific HTML inline tag with an optional CSS class to the selected text.

The plugin includes a built-in nesting validation system: buttons are automatically disabled when their use would produce invalid HTML nesting (e.g. <a> inside <a>). When a format is already active, its button remains enabled so it can always be toggled off.

Key features:

Register custom inline formats (tag + CSS class) via a WordPress filter
tagName defaults to span, which is strongly recommended for compatibility
Smart nesting validation against the full active tag stack (for non-span tags)
Buttons are visually disabled when nesting is not allowed
Exhaustive inline tag ruleset (span, a, strong, em, mark, code, u, s, i, b, q, cite, abbr, small, sub, sup)
Inline tag rules are filterable for full customisation by themes or other plugins

For developers:

Formats and inline tag rules are both exposed via WordPress filters, making the plugin fully extensible without modifying core files.