Source code: https://github.com/germain-italic/wp-highlight-string-on-page
A WordPress plugin with Shortcode (works with old editor):
Demo text:
Proin viverra, ligula sit amet ultrices semper, ligula arcu tristique sapien, a accumsan nisi mauris ac eros. Vestibulum ullamcorper mauris at ligula. Praesent egestas tristique nibh. Suspendisse feugiat. Maecenas tempus, tellus eget condimentum rhoncus, sem quam semper libero, sit amet adipiscing sem neque sed ipsum.
Sed consequat, leo eget bibendum sodales, augue velit cursus nunc, quis gravida magna mi a libero. Suspendisse pulvinar, augue ac venenatis condimentum, sem libero volutpat nibh, nec pellentesque velit pede quis nunc. Quisque ut nisi. Praesent vestibulum dapibus nibh. Quisque ut nisi.
How to install the plugin?
Extensions > Add extension
Upload ZIP from https://github.com/germain-italic/wp-highlight-string-on-page/releases/download/v1.0/wp-highlight-string-on-page-v1.0.zip, install, activate.
How to display the widget?
Insert the following Shortcode where you want to display the widget: [ highlight_search ] (remove the spaces in the brackets)