In brief
opacity affects the entire element together with its contents. At opacity: 0, it can remain in layout, receive pointer events and participate in keyboard focus.
Useful to know
opacity affects the entire element together with its contents. At opacity: 0, it can remain in layout, receive pointer events and participate in keyboard focus. [1]
Transparency alone is therefore not a reliable way to disable a hidden button. For a translucent background only, use a colour with an alpha channel rather than fading the text too.
Sources
For further detail and to check the information: