The :dir() CSS pseudo-class selector matches elements based on directionality, which is determined based on the HTML dir attribute. :dir(ltr) matches left-to-right text directionality, and :dir(rtl) matches elements with right-to-left text directionality. It is not equivalent to [dir] attribute selectors because it matches against directions inherited from an ancestor with the dir attribute, and because it matches against the direction computed from use of dir=auto (which determines directionality from the first character in the text with strong directionality).
Samples: https://jsfiddle.net/fxc9a8uc/1