This is one of those things that won't stay in it's allocated memory slot. This time I'm going to write it and then forget where I wrote it instead.

Using a placeholder to create a comparison slider, but the same applies to most of these things. Where does the following go? Page, Website, Placeholder, Body, Head???

HTML?
CSS?
JS?

The example I'm working on >>> Before After Image Slider (Vanilla) (codepen.io) <<<