• When you want to hide from AT (assistive tech, like a screen reader) (like an irrelevant modal)
• When you don't want to hide from AT (like a dropdown menu)
• When you want to keep in flow
• When you want to remove from flow
• When you want to collapse from flow
• Dealing with clickability

Opacity / Visibilty / Display

https://css-tricks.com/video-screencasts/142-hiding-things-with-css/

https://css-tricks.com/places-its-tempting-to-use-display-none-but-dont/

https://accessibility.18f.gov/hidden-content/

https://a11y-guidelines.orange.com/web_EN/exemples/masquage/index.html