Some languages, such as Arabic and Hebrew, are written from right-to-left, while keeping some characters are written (like numbers) left-to-right. Therefore, a browser that supports such languages is said to have BiDi (bidirectional) support.
BiDi support has been part of the HTML standard since version 4, which was released 18 December 1997 (and was updated 24 April 1998) , using the DIR attributes,
As of Opera 7.2, BiDi support is in! :) However, much work is still to be done.