This could be the result of a number of things, the first, and most likely being that a special character was pasted into the document from Windows-document. For Web pages to display accented characters and the like, they must be properly encoded.
Windows has its own character set, which should not be used for Web pages.
Another reason could be the result of older Macintosh browsers (Netscape and Internet Explorer versions 4 and below). The Macintosh’s default character is missing 14 items from the standard ISO Latin-1 set. Among those missing, the fraction and the superscript characters.
For more information read Bob Baumel’s comprehensive article, “Using Special Characters on Web Pages: Cross-Platform Considerations.”