The pseudo-select :is() in CSS allows you to write compound selectors more effectively.
Simplifying selectors with :is() is similar to how CSS preprocessors handle nested rules.
The specificity of the :is() pseudo-class is replaced by the specificity of its most specific argument. Thus, a selector written with :is() does not necessarily have equivalent specificity to the equivalent selector written without :is(). if any part of a selector is invalid, the entire block is thrown out.
The :invalid selector allows you to select input elements that do not contain valid content, as determined by its type attribute. :invalid can be “chained” with other pseudo-selectors: like :focus to only validate when the user is typing, :before or :after to generate icons or text to provide more user feedback.
The :valid selector allows you to select input elements that contain valid content, as determined by its type attribute.
This selector has one particular use: providing a user with feedback while they are interacting with a form on the page.
:indeterminate is a pseudo-class selector in CSS named for a state that is neither checked nor unchecked. It’s that in-between state that we might consider the “Maybe” between “Yes” and “No” options. The state is not fully determined, hence the name: indeterminate.
there is no way to set a checkbox to an indeterminate state in HTML. :indeterminate can apply to radio buttons at the group level, where the entire group is considered to be in an indeterminate state if no option is selected.