Jonas Grønås Drange

The plugin adds one js-file which searches through your blog and adds extra HTML markup to <code>.key-shortcut</code> elements.
A stylesheet is added that applies styling to the extra HTML markup.
This results in <code>.key-shortcut</code> elements looking somewhat like keyboard buttons.

Key Shortcut Formatter

Key Shortcut Formatter enhances the presentation of keyboard shortcuts with simple CSS and JavaScript

Rolf Allard van Hagen

Highlights search terms within WordPress generated search results, both on the search results page and on each linked post page itself.
This plugin is light weight and has no options. It started as very simple fusion between <a href="http://weblogtoolscollection.com/archives/2009/04/10/how-to-highlight-search-terms-with-jquery/" rel="nofollow ugc">How to Highlight Search Terms with jQuery – theme hack by Thaya Kareeson</a> and <a href="https://wordpress.org/extend/plugins/google-highlight/" rel="ugc">Search Hilite by Ryan Boren</a>. It has since evolved with many optimizations, HTML5 and bbPress support.
Since version 1.6 it no longer depends on the jQuery library.
Features
<ul>
<li>Click through highlights: Highlights not only on WP search results page but also one click deeper inside any of the found pages</li>
<li>Character and case insensitive (lenient) highlighting</li>
<li>BuddyPress / bbPress compatibility: highlighting within forum searches</li>
<li>Caching (WP Super Cache) compatibility</li>
<li>Search terms wrapped in double quotes now considered as single term</li>
</ul>
<h4>What does it do?</h4>
This low impact plugin finds all search terms on a search results page inside each post and highlights them with a <code> ... </code> tag. 
Note that N is a number starting with 0 for the first term used in the search phrase increasing 1 for each additional term used. Any part of a search phrase wrapped in quotes is considered as a single term.
<h4>What does it NOT do?</h4>
There are no CSS style rules set for highlighting. You are free to use any styling you wish but to make the highlights visible in browsers that do not support HTML5 like Internet Explorer 8 or older you absolutely need to define at least one rule. 
Modern HTML5 browsers will use their own highlighting style by default, which usually is a yellow marker style background.
<h4>So what do I need to do?</h4>
In most cases, it should just work. But you can do two things to ensure backward browser and theme compatibility:
<ol>
<li>
Define CSS rules: There are no configuration options and there is no predefined highlight styling. You are completely free to define any CSS styling rules in your themes main stylesheet (style.css) or the Custom CSS tab if the WordPress theme customizer. 
You can find basic instructions and CSS examples in the <a href="https://wordpress.org/plugins/highlight-search-terms/#faq" rel="ugc">FAQ’s</a>.
</li>
<li>
Check your theme: In most up to date themes (including WP’s own default theme) post and page content is shown inside a div with class <code>hentry</code>. This means search terms found in post and page content will be highlighted but not similar terms that accidentally show in the page header, sidebar or footer. 
If your current theme does not use the <code>hentry</code> class (yet), this plugin will look for IDs <code>content</code>, <code>main</code> and finally <code>wrapper</code> but if none of those are found, it will not work for you out of the box. See the last of the <a href="https://wordpress.org/plugins/highlight-search-terms/#faq" rel="ugc">FAQ’s</a> for ways to make it work.
</li>
</ol>
<h4>Available hooks and filters</h4>
<ol>
<li>
<code>hlst_query_vars</code> – The array of WordPress query variables that the plugin will identify as a search query. Must return an array. Default: <code>['search_terms','bbp_search']</code> (WordPress abd bbPress search)
</li>
<li>
<code>hlst_input_get_args</code> – An array of GET variables that the plugin will identify as a search query. Must return an array. Default: <code>['hilite']</code> (for click-through highlighting)
</li>
<li>
<code>hlst_selectors</code> – The array of possible HTML DOM element identifiers that the script will try. The first viable identifier it finds elements of will be scanned for search terms to mark, the rest is ignored. So the order is important here! Start with the element closest to, but still containing all the post/page title, excerpt or content.
</li>
<li>
<code>hlst_events</code> – The array of DOM event listeners that the inline script will watch for. Default: <code>['DOMContentLoaded','post-load']</code> (on Document Ready and for Jetpack Infinite Scroll and others).
</li>
<li>
<code>hlst_inline_script</code> – The inline script that will be added to the plugin script file. Can be used to add to or alter the inline script. Must return a string.
</li>
</ol>
<h4>Known issues & development</h4>
<ol>
<li>
If your theme does not wrap the main content section of your pages in a div with class “hentry” or HTML5 article tags, this plugin might not work well for you out of the box. However, you can make it work. See the last of the <a href="https://wordpress.org/extend/plugins/highlight-search-terms/faq/" rel="ugc">FAQ’s</a> for an explanation.
</li>
<li>
<a href="http://theveganpost.com" rel="nofollow ugc">Josh</a> pointed out a conflict with the ShareThis buttons plugin. Since then, that plugin has been completely rewriten so please let me know if the problem still exists. Thanks!
</li>
</ol>
Please file bug reports and code contributions as pull requests on <a href="https://github.com/RavanH/highlight-search-terms" rel="nofollow ugc">GitHub</a>.

Highlight Search Terms

Absolutely perfect

Very lightweight (vanilla) Javascript that wraps search terms in an HTML5 mark tag within WordPress search results.

Key Shortcut Formatter vs Highlight Search Terms