[WordPress] Through a combination of the TinyMCE API and the HTML5 mark tag, here's how you can highlight text in the WordPress editor.

00-add-tinymce-button.js

init: function( ed, url ) {

	ed.addButton( 'highlight', {

		title:   'Highlight Text',
		cmd:     'highlight',
		image:   url + '/img/highlighter.png'

	});

}

01-wrap-the-text.js

ed.addCommand( 'highlight', function() {

	var markOpen  = '<mark>',
	    markClose = '</mark>',
	    highlight = markOpen + ed.selection.getContent() + markClose;

	ed.focus();
	ed.selection.setContent(
		markOpen + ed.selection.getContent() + markClose
	);


});

02-toggle-the-text.js

ed.addCommand( 'highlight', function() {

	var markOpen  = '<mark>',
	    markClose = '</mark>',
	    highlight = markOpen + ed.selection.getContent() + markClose;

	ed.focus();

	/* Determine if there is highlighted text.
	 * If so, remove it; otherwise, highlight it.
	 */
	if ( -1 < ed.getContent().indexOf( highlight ) ) {

		ed.setContent(
			ed.getContent().replace( highlight, ed.selection.getContent() )
		);

	} else {

		ed.selection.setContent(
			markOpen + ed.selection.getContent() + markClose
		);

	}


});

03-add-editor-style.php

<?php

add_action( 'admin_init', 'acme_add_editor_style' );
/**
 * Adds a stylesheet to provide styling the for WordPress Editor
 *
 * @since    1.0.0
 */
function acme_add_editor_style() {
    add_editor_style( get_template_directory_uri() . '/assets/css/editor-style.css' );
}