Nowadays, displaying and inputting Japanese on the Web is considerably easier than it was a few years ago. We used to maintain an extensive discussion of how to access Japanese character texts on the Web for the Japanese Text Initiative (JTI). Now the recent versions of Microsoft Internet Explorer and Microsoft's "Input Method Editor" (IME) for Japanese make accessing Japanese on the Web easy.
The following sites offer help with accessing Japanese:
Alan Wood's Unicode Resources: Focuses on how to use Unicode, but includes extensive discussions of topics like browsing multilingual Web pages with Microsoft Windows and Apple Macintoshes.
For older operating systems: The Japanisation FAQ for Computers Running Western Windows.
Jim Breen's indispensable list of resources and links for Japanese computing, especially his section on Computing in Japan/Japanese.
Unicode: You need a Unicode font such as Arial Unicode MS. For an excellent discussion of Unicode fonts, see Alan Wood's site above. Without a Unicode font you can display some characters but not necessarily all. For example, on some versions of Windows, Japanese characters can be displayed if you have a Japanese font installed. But characters with macrons in Romanized texts usually cannot be displayed without a Unicode font. On Macintoshes with older operating systems, macron characters may require installation of the central European language kit. See Alan Wood's site for details.
Furigana (Ruby): Furigana can be displayed with recent operating systems and Internet browsers. In general, for Windows, you need Windows 2000 or XP and Internet Explorer 5.5 or later. As of January 2006 there is an add-on for Mozilla Firefox that supports furigana (ruby). Other browsers, like the latest version of Netscape, may not display furigana correctly. For Macintoshes furigana can be displayed in OS 9 or higher and Internet Explorer 5.1 or higher. If you do not have recent systems and browsers, furigana in the JTI texts will appear in parentheses in the body of the text.
Vertical Display: The JTI is now making some of its texts available in both horizontal and vertical displays. As with furigana, you need recent operating systems and browsers in Windows in order to display vertical text. The vertical display provided by the JTI is unfortunately not yet supported in Mozilla Firefox or in Macintoshes.