The Unicode standard governs how modern software handles textual data, including characters from all of the world's natural languages, as well as thousands of emojis and other pictorial symbols ― over 110,000 characters in all.
A further complication is that some emojis, such as ❤️, are composed of two adjacent Unicode characters. (That's a red heart. If your browser isn't fully compliant, you might see it as a black heart, ❤).
Luckily, the Unicode standard fully specifies how all of this works, and the rules are simple to implement in code.
You can also enter numeric values, if you select the "Numeric" checkbox. The following formats are accepted:
These numeric values can be strung together, to form:
unicode-utils-js on Github.