Opera is basically a play in which the actors sing their lines. The play can be any type, from tragedy through comedy to downright slapstick. Because opera is a form of classical, though, the music (the "score") and lyrics (the "libretto") follow a specific structure.
As with all other forms of classical music, you may find some experimental opera here. Remember from grammar lessons that you have to know the rules before you can break them? Same is true for classical -- experimental, or alternative, classical artists first study and understand the underlying rules and structures. Those who don't may create a type of music, but not it's not classical.
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