In this snippet by Tommy Hodgins we can see the effect where the entire page is behind a fog of war like effect and is barely visible. When you move the mouse around you can see that there is a spotlight like effect, that reveals the content in that area, it kinda feels like pointing a flashlight onto the web page.
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Mouse Cursor as Flashlight! 🖱🔦 using CSS Variables by Tommy Hodgins (@tomhodgins)