dsmmcken has shown that making a full blackout with CSS is possible with clip-path, but at the time I wrote this, I didn't know that.

The CSS comes in the form of the mix-blend-mode divs, "grayscale", "bright white", "inverse", etc. They are more like lenses than flashlights since they don't illuminate what's not already there, but flashlights were my inspiration for doing this so that's the term I stuck with, though I did use the word lens once.

The CSS idea came first, and the canvas idea came second when I wanted to make a full blackout and didn't know about clip-path.