The language of computers used to be English. Support for other languages was added over time. Until recently, it wasn’t easy to display many of the national alphabets and scripts. East Asian languages have been supported for some time already, but furigana was added only recently.
Right now, in April 2015, most modern browsers support furigana using something called ruby annotations. If it works in your browser, you should see it here:
You can use a Firefox add-on called HTML Ruby in the meantime. However the toggle button doesn’t work properly. You will have to use the add-on itself to turn the display on or off.
Ruby can be used to add annotations above text. Typical use is adding furigana to Kanji. I’ve added a little switch (Toggle furigana, try it!) that you can use to turn off the furigana and try to read the text without it. Also when you want to copy the text to let’s say Google Translate you need to first turn off the furigana. Otherwise furigana gets copied inside the text. Let me know in the comments if you like this feature.