Then I thought to myself... I know you can't add Furigana to online manga because they are images, but I wonder if you can add Furigana to online text. So after about 30 seconds of searching I came across this Firefox add-on called Furigana Inserter & Chrome extension called Furigana Injector that does just that:
As you can see it places Furigana over most of the Kanji which is awesome. The only thing is that the lowest ignore setting you can put it on is "Ignore 100 most common Kanji" so things like 日本 will not have Furigana. Then again you can always use Rikai-Chan for Firefox or Rikai-Kun for Chrome to get the reading of the more common Kanji.
Happy web surfing!