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.

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 …

Quicktags for WP vs Easy Tinymce Editor Add Button