Sorry if you think I'm authoritative, I didn't mean to be.
You might consider revising your style in the future. Entering a discussion by saying, "I can't believe what stupidities you people are babbling in this forum!" is usually not a way to win much sympathy.
The source for everything I wrote is this: I have lived in Croatia for 25 years, I finished High school here, I had Croatian in High school, I had Croatian as a subject on my Final examination (Matura). I am a college student, my roommate is a professor of Croatian language and literature. Guys, I speak Croatian 24 hours a day, 7 days a week, ... I listen it on TV, read newspapers, books... I know that "da li" is wrong. I don't hear it, I don't see it.
The thing is this: In this thread you allege that I am wrong, but how can you say that? One of you lives in Toronto, and the other one in Serbia.
I live in Croatia. Don't write wrong facts in this forum, it is supposed to be objective and scientific.
"Location" refers only to the present physical location of each user. I also finished high school and university in Croatia, I lived there until only a couple of years ago, and I might easily move back there soon. I still visit Croatia regularly and use Croatian 7 days a week, both written and spoken, so I don't think these few years abroad have reduced my Croatian language skills in any way. I've produced many posts in Croatian in this forum, so feel free to use them to judge the quality of my Croatian. From you, we've seen so far only a small, badly punctuated piece of substandard slang (and you're the one pontificating on language purity here!).
dudasd, you say that if that were true, 90% of Croatian writers would be illiterate. Language changes. I never said that "da li" has always been wrong. In post WW2 Croatian it is 100% wrong.
But well over 90% of Croatian writers, both living and dead,
are illiterate, or at least hopelessly Serbianized, if you apply to them the criteria for purity like this that are pushed by the regular ilk of Croatian purists. Yes, of course that language changes, but not according to the dictate of a bunch of linguistically clueless nationalist ideologues who have the hubris to proclaim themselves arbiters of what constitutes valid Croatian language (for which they don't even have any sort of official sanction, let alone any realistic qualifications).
According to what you write, when I read the classics of Croatian literature, I am supposed to first screen their work against the dogmatic pronunciations of modern purists before I decide to enrich my own language with the vocabulary and expressions that they used. Sure, older writers have used some obviously archaic language that it would be ridiculous to use nowadays, but archaic expressions are supposed to die a natural death, not to be legislated out of existence. I mean, what hubris does it take to issue proclamations that something was a valid part of the language until a certain point in time, but no longer? And here I'm not talking about minor reforms of spelling or punctuation, but about anathematizing whole swaths of vocabulary and grammar -- in this case, one that is still alive and kicking in the modern language (see below).
You cannot find it ["da li"] in newspapers, hear it on TV, hear it anyone saying.
You say you've lived your whole life in Croatia, and you've never heard that song by
Majke --
"Ja sam budućnost, da li ti se sviđam?" 
Should the lyrics of the best Croatian rock'n'roll band of all time be purged from the Croatian literary tradition? After all, they're full of not only questions starting with "da li", but even (gasp!

) sentences such as
"ne želim da razumijem"!
Let's try googling for "da li je" on various Croatian media websites (mind you, this is only one out of a myriad possibilities for starting questions with "da li"!):
- Vjesnik (a major political daily newspaper): 1,760 hits
- HRT (Croatian national TV): 1,310 hits (that's excluding the forum section of their website!)
- Nacional (a major political weekly magazine): 610 hits
- Glas Koncila (a Catholic newspaper): 21 hits
- Novi list (another major daily newspaper): 434 hits
I think this is a fair sample of Croatian mainstream media. Obviously, you must be living in a state of extreme isolation in Croatia if you never hear or read questions starting with "da li"!
Admittedly, most of these hits refer to the use of "da li" in indirect speech, although on each of these websites, there are also at least some examples of its use in direct speech. And anyway, from what you write, it seems to me that your blanket condemnations of "da li" include indirect speech too.