:invalid is a pseudo-class selector that enables styling on invalid form elements. :invalid can be added to any form elements, […]

form:invalid { outline: 3px dashed red; }

The :in-range pseudo selector in CSS matches input elements when their value is within the range specified as being acceptable. […]

input:in-range { border: 5px solid green; }

:indeterminate is a pseudo-class selector in CSS named for a state that is neither checked nor unchecked. It’s that in-between […]

input[type="radio"]:indeterminate { }

The pseudo-select :is() in CSS allows you to write compound selectors more tersely. For example, rather than writing: We could […]

:is(ul, ol) li { color: #f8a100; }