HTML5 has several new features designed to help web designers create pages that show RTL text properly. Among them is the new <bdi> element. It stands for "bi-directional isolation", and it basically means that the text inside it doesn't mix up with adjacent punctuation marks and numbers, which have ambiguous directionality properties.
When it will be widely supported in browsers, this element will become very useful in Wikimedia projects. Probably the most common use case for it is showing the left-to-right name in an article about a person in a right-to-left Wikipedia; this name is usually written in the opening paragraph and adjacent to the birth date, which consists of numbers that get mixed up because of the bidi algorithm. Currently it is solved by using Unicode control characters (RLM), <bdi> is a more elegant solution.
This tag is currently sanitized, so it is impossible to use it in MediaWiki pages. It shouldn't be sanitized. To do this, i suppose that it should be added to htmlpairsStatic in includes/Sanitizer.php, but i'm not so familiar with that class, so maybe something else is needed.
When will it be widely supported in browsers? It was just added to Mozilla, and note that even the source file name is similar:
AFAIK, it is also about to be added to Chrome.