:indeterminate CSS pseudo-class

The `:indeterminate` pseudo-class matches indeterminate checkboxes, indeterminate `<progress>` bars, and radio buttons with no checked button in their radio button group.

css property: appearance: progress-bar

css property: direction: control direction of vertical range sliders, meters, and progress bars

css selector: -moz-progress-bar

css selector: `::-webkit-progress-bar`