Fokus
Fokus uses JavaScript to emphasize anything you select by covering the rest of the page with semi-transparent black.
Try it out by selecting this paragraph or the sample content below. You'll see the entire page fade out while this text is highlighted.
If you want to use Fokus on your site you just need to include the fokus.min.js script (3kb, no dependencies).
Fokus is also available as a Chrome extension.
Created by @hakimel