But as is his wont, he boasted about his computer skills.
Is the sentence above grammatical? And if so, is "his wont" a subject in the subordinate clause But as is his wont?
Yes, it's grammatical. g. "as is his habit").
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Yes, it's grammatical. "as is his wont" is a kind of a set phrase (though the pattern works with other nouns, e.g. "as is his habit"). I would say that it is an inversion of "as his wont is" (though that word order is not so idiomatic). Therefore, yes, I would say that "his wont" is the subject of the verb "is".