cobianzo

This is a very simple plugin that allows you to associate a tooltip to any link in the block editor. 
Open the link options, click on the pencil icon to edit it, then make sure that the Advanced panel 
is open, and find the new input box for the tooltip. 
You will see the tooltip when you hover with the mouse the link in the frontend.
Main features: 
– new ‘tooltip’ input box in the CMS editor ‘edit link’, under ‘Advanced’ tab. 
– showing this new field as a simple CSS-only tooltip in the frontend 
– CSS rules use some variables to easily modify the aspect of the tooltip.

GB CSS Tooltip

Simple functionality that includes an extra option when editing a link within the block editor: nice CSS tooltips.

Nico Danneberg

Adds explaining tooltips querying their content from a <a href="https://www.mediawiki.org" title="see MediaWiki docs" rel="nofollow ugc">MediaWiki</a> installation, e.g. <a href="https://www.wikipedia.org" title="see the well-known Wikipedia" rel="nofollow ugc">Wikipedia.org</a>. Therefore shortcodes can be used in Posts and Pages to mark keywords and link them to public Wiki pages. The well-known package of <a href="https://calebjacob.github.io/tooltipster/" title="Tooltipster rocks :)" rel="nofollow ugc">Tooltipster</a> is used to create the nice and themable tooltips.
Main features of the current version are:
<ul>
<li>Setup at least one wanted Wiki base and several other options at a backend page</li>
<li>Integrate the Wiki tooltip in Posts and Pages using a handy popup in Gutenberg editor or simple shortcodes</li>
<li>Shortcodes can be created by a <a href="https://codex.wordpress.org/TinyMCE" rel="nofollow ugc">TinyMCE</a> plugin, too</li>
</ul>

WP Wiki Tooltip

Adds explaining tooltips querying their content from a MediaWiki installation, e.g. Wikipedia.org.

GB CSS Tooltip vs WP Wiki Tooltip