Emoji are “picture characters” originally associated with cellular telephone usage in Japan, but now popular worldwide. The word emoji comes from the Japanese 絵 (e ≅ picture) + 文字 (moji ≅ written character).
Emoji are prominently used in reacji responses.
- Aaron Parecki includes emoji characters in note and photo contents, and outputs the raw emoji characters in the HTML rather than replacing them with images. Examples:  
Emoji are rendered differently on different platforms, with some examples leading to differing interpretations of the same emoji, depending on how they are displayed to the user (see also articles , ).
standardized vs platform-specific display
Many silos standardize their emoji display by replacing them with the same image on all platforms. (e.g. WhatsApp, Twitter, Telegram, Threma). Replacing them with images also allows to show colored emoji everywhere, which is very difficult using fonts, since there are conflicting standards for multi-colored glyphs. Several permissively-licensed image sets are available, e.g. by Twitter, Emoji One and Google. If a site replaces emoji with images, it should set the alt="" attribute to the text version of the emoji.
https://eev.ee/blog/2016/04/12/apple-did-not-invent-emoji/ Good overview over the history and differences