From googling this is apparently an issue with fonts not having the appropriate Unicode support for the vast array of different symbols associated with them. So whatever font your phone, PC, and I'm guessing that extends to webpages as well, are incapable of encompassing all of the Unicode symbols/emoji's that exist. From what I read it's not possible to have a font that's capable of displaying every Unicode symbol, because they can't fit them all into one font, and the number of symbols continues to grow. I'm not sure how it relates to webpages and browsers. I'm guessing the webpage would supersede the browser when displaying stuff on the page, but I'm not sure if that's true. I.e. it wouldn't matter what fonts you were using in the browser if a webpage had it's own specific font it was using, so your browser would potentially be able to display the Unicode, but the webpage wouldn't. I don't quite know how it all works though, so this could be completely wrong.