UTS #51 is your friend.

There's not really a "standard" character set and an "emoji" one. There is an emoji block (U+1F600 to U+1F64F), and this heart (U+2665) is not in that. But this heart is an emoji, in that the data file emoji-data.txt for contains `Emoji=Yes` ("=Yes for characters that are emoji", —UTS #51) for this character. However, this heart has `Emoji_Presentation=No`, which "=Yes for characters that have emoji presentation by default" (—UTS #51).

So basically, it should present, by default, in its non-emoji appearance. (Though there seem to be a number of readers for whom that isn't true. I think that's macOS being badly behaved.) There are some characters that can flip/flop between a "text" (normal monochrome) presentation and an emoji presentation; all of the card suit characters are. Unadorned (e.g., just U+2665 alone) then, you have to follow the emoji default presentation rules. In some contexts, such as web pages, that's "just do what that Emoji_Presentation flag says" for those characters, but see [§Presentation Style](https://www.unicode.org/reports/tr51/proposed.html#Presentat...) for exceptions to the rule. (HN is not an exception.) But you can also follow the character with a variation selector, to try to force either text (U+FE0E) or emoji (U+FE0F) presentation.

There are a number of what I'd call "yeah that's definitely an emoji" outside the Emoji block, too. Like U+2705 "white check in green box" (which HN disallows me posting); that one is Emoji=Yes but also Emoji_Presentation=Yes)

(IDK if Emoji=Yes Emoji_Presentation=Yes is HN's filter … but that'd be what I would do, if I wanted to filter out emoji like HN does.)

UTS #51: https://www.unicode.org/reports/tr51/proposed.html#Emoji_Pro...

(Also, note that the title was changed; the original HN title was "I ♥ RSS")