A couple of birds with one stone:
- Use masking to selectively alter brightness & contrast
- Use a before-after slider to illustrate the differences
Update from original slide post – the broken slider is fixed. For anyone interested (probably no one) the problem was caused by a WordPress plugin (you’d thing WP, of all people, would get things right). The plugin, Jetpack, has an option call Lazy Load. Problem went away after I disabled this option.