I like how when the voice says "¡Feliz Navidad!" and you're supposed to type what she said, so you write "Merry Christmas!" (which is what she said but hey) and they come back saying "Oops that doesn't seem to be Spanish." (without taking away one of your precious hearts) Am I the only one who does this?
99.9% is quite an exaggeration, although if you think it should be accepted, then simply use the report button (but I'm sure you've done that). Personally I think it's not a good habit to be writing improper terms when there are people learning English here (doing reverse courses) but no hard feelings, if you use it over Christmas, then report it.
I suppose it depends on how you define an "improper term". DuoLingo accepts many abbreviations such as "it's" or "let's"; Xmas has been used since the 16th century so it's got to be as valid as any of the others.
As an aside, just as you pronounce "mr." as mister, you pronounce Xmas as Christmas.
It's recognised by everyone but there is no way in hell that 99.9% of people use it. I'd go the other way and say that most people use "Christmas". It's nearly always what is used on Christmas cards, and it's pretty much the only way it's said in speech (Have you ever heard a person say merry xmas?). The only places I ever see "xmas" is in text messages and on Facebook.
Using the word "xmas" as a replacement/substitution for the word "Christmas" is quite unnecessary. Since the word "Christmas" is not old, outdated, lacking full meaning, grammatically incorrect or a dead word, it makes no sense whatsoever why "xmas" is even needed. If a word is still currently being used in the English language, (such as the word Christmas) there is no need to implement a substitution word in it's place. It would be like if someone decided that the letters"DEC" at the start of the word "December" were no longer necessary, so they changed the word to "Xember." Now that itself would be ridiculous, as the word "December" is used in every day language, is not outdated, and has full grammatical meaning. Now imagine that the companies that make calendars caught on to this and started printing on their calendars that the 12th month of the year was called "Xember". Of course very few (if any) people would buy this calendar, because it is a completely unnecessary change of the word from "December" to "Xember". It's the same with "Christmas" and "xmas". There are no grounds and no basis to change the word, therefore it should be eliminated from professional environments. (such as Duolingo, which is trying to teach you a new language, not a new form of English slang) That being said, I see no reason why Duolingo should allow "xmas" to be used as a proper translation.