RTL mirroring

Arabic, Hebrew, Persian and Urdu are read from the right, and a screen in one of those languages is not the same screen with the text right aligned. It is the mirror image: the navigation starts on the right, the back control points the other way, a progress bar fills leftward, a sidebar swaps edges, and the first cell of a table is the rightmost one. Setting dir="rtl" on the document is the switch that says so, and everything after that is a question of whether the layout was written in a way that can hear it.

Almost all of the work is avoided rather than done, by writing the layout in logical terms from the start. margin-inline-start instead of margin-left, inset-inline-start instead of left, padding-block instead of top and bottom, text-align: start, and border-start-start-radius for the corner nearest where reading begins. Flexbox and grid already work this way, since a row runs along the inline axis and reverses with it, which is why a layout built from them and from logical properties often mirrors correctly the first time it is asked to. Every physical value left in the sheet is a small hardcoded assumption about which side the reader starts on, and they are much easier to avoid than to find later.

What does not mirror is where the judgment lives, and it comes down to what an element is about. Anything that represents reading order or forward progression flips: back and forward arrows, undo and redo, indentation, the direction a stepper advances. Anything that represents a physical object or a shared convention holds: a clock face, a checkmark, a star, media playback controls, a company logo. Digits are the case that surprises people, because numbers run left to right even inside right to left text, so a time or a measurement keeps its own internal order while moving to the other side of the row. The practical form of this is an icon audit: go through the set once, mark each glyph mirror or hold, and keep the list, because the answer for a given glyph is a decision rather than something a stylesheet can derive.

The second half of the term is the part the name only hints at. Real content is bidirectional rather than uniformly one direction: a Latin brand name inside an Arabic sentence, an email address in a Hebrew paragraph, a user-supplied string of unknown direction dropped into your layout. The Unicode bidirectional algorithm resolves most of it, and the tools for the rest are dir="auto" on inputs and user content, and <bdi> to isolate a run so a stray strong character cannot reorder the text around it. Test with real strings in a real language, since the reliable way to find the one hardcoded left is to look at a screen where everything else has already moved.