This is a 100% Opaque element, everything inside this tag is fully visible

This is a 60% Opaque element, you notice that this text is hard to read beacuse Opacity affects the entire elelemt and all the content inside it.

This is an element that turns more opaque on hover.

This is an element has a transition effect, put the mouse over and the opacity will increase from 0.4 (40%) to 1 (100%) in 1 second.