trubine

Simple plugin for adding buttons to the html wp panel of the tinymce editor.
Features:
<ul>
<li>Ease of use</li>
<li>Ability to add any content</li>
<li>Unlimited number of buttons</li>
<li>Support for strings / tags / shortcodes</li>
<li>Ability to add both paired and single shortcodes / tags</li>
</ul>

Easy Tinymce Editor Add Button

Simple plugin for adding buttons to the html wp panel of the tinymce editor. Features: Ease of use Ability to add any content Unlimited number of b …

David Klhufek

This simple plugin adds useful buttons to the native WordPress HTML editor (Classic Editor).
Currently adds: 
h1, h2, h3, h4, h5, span, hr, nbsp, mox
The “mox” button adds a nice responsive CSS class which you can use to highlight pictures or other elements in your posts.
Note: This plugin works only with the Classic Editor. It does not support the new Gutenberg (Block Editor).
<h3>Compatibility</h3>
<ul>
<li>Requires the Classic Editor to be active. </li>
<li>Designed specifically for the HTML mode of the editor.</li>
</ul>

Quicktags for WP

This simple plugin adds useful buttons to the native WordPress HTML editor.

Easy Tinymce Editor Add Button vs Quicktags for WP