( new mw.Title('ß') ).getPrefixedText() does not round-trip in some browsers. This could render the article https://en.wikipedia.org/wiki/ß inaccessible by some JavaScript tools. (See also T141723.)
The correct result is 'ß', incorrect results are 'SS' or 'ẞ'. Chromium 54 gives me 'SS', Firefox and Edge are OK.