Aki Hamano

<p>RichText Extension adds useful decorating features to gutenberg rich text editor.</p>
<h4>Highlighter</h4>
<p>Draw a marker on the inline text.<br />
You can set the color, thickness, and opacity of the marker to you can create various marker styles.<br />
Up to four types of settings can be made.</p>
<h4>Font size</h4>
<p>Change inline text size.<br />
The size is specified as a percentage of the base font size, and up to four types of settings can be made.</p>
<h4>Underline</h4>
<p>Create underline.<br />
<strong>Note: The underline specifications have changed from version 2.0.0. Try clearing the format if existing underlines do not work.</strong></p>
<h4>Clear format</h4>
<p>Removes all formatting.</p>


RichText Extension

This is by far the best approach I've seen so far on really adding something usable to the WP Gutenberg Block Editor

well done ...

Adds useful decoration features to the Gutenberg RichText editor toolbar.

Ju'Z[ed]

<p>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.</p>
<p>The plugin includes a built-in nesting validation system: buttons are automatically disabled when their use would produce invalid HTML nesting (e.g. <code><a></code> inside <code><a></code>). When a format is already active, its button remains enabled so it can always be toggled off.</p>
<p><strong>Key features:</strong></p>
<ul>
<li>Register custom inline formats (tag + CSS class) via a WordPress filter</li>
<li><code>tagName</code> defaults to <code>span</code>, which is strongly recommended for compatibility</li>
<li>Smart nesting validation against the full active tag stack (for non-span tags)</li>
<li>Buttons are visually disabled when nesting is not allowed</li>
<li>Exhaustive inline tag ruleset (<code>span</code>, <code>a</code>, <code>strong</code>, <code>em</code>, <code>mark</code>, <code>code</code>, <code>u</code>, <code>s</code>, <code>i</code>, <code>b</code>, <code>q</code>, <code>cite</code>, <code>abbr</code>, <code>small</code>, <code>sub</code>, <code>sup</code>)</li>
<li>Inline tag rules are filterable for full customisation by themes or other plugins</li>
</ul>
<p><strong>For developers:</strong></p>
<p>Formats and inline tag rules are both exposed via WordPress filters, making the plugin fully extensible without modifying core files.</p>


JuZ Rich Text Extender

Extends the Gutenberg block editor toolbar with custom inline format buttons, with smart nesting validation based on HTML spec rules.

RichText Extension vs JuZ Rich Text Extender