:where() CSS pseudo-classes from the Selectors Level 4 spec have been enabled with Firefox 78. In Firefox,
:is() had been implemented as
:-moz-any() for a long time. The prefixed pseudo-class is still available but may be removed in the future. Make sure to use both the standard and non-standard versions to future-proof your website.
How can I test this?
This change can be tested with the compatibilty checker in our Firefox Developer Tools extension. Get it today!