While arts writing is going through one of its richest periods of innovation, with an explosion of forms in recent years, much of the experimentation is happening well outside of traditional media. The internet seems to have reminded at least some writers of the kind of artistry that’s possible in art criticism, says Charlotte Frost, author of a forthcoming book, “Art Criticism Online: A History.” This represents a return to the roots of the field, she adds. The 800-word art review is actually a fairly recent invention. But if you turn the clock back a bit, to the 18th-century Paris salons, for instance, there were all kinds of critical responses to art, Frost says.