An informal writing style in nonfiction would contain
a serious tone
numerous factual presentations
a witty, chatty approach
a detailed critique of the object

An informal writing style in nonfiction would contain - a witty, chatty approach. If we are writing something informal, that means that we don't have to follow some specific rules. Our tone doesn't have to be serious, we don't have to include all important information about the topic, and we don't have to give detailed critique of the object. We can just say what we think, and it is not necessary to give arguments. Also, our style can include jokes and wits which can help the reader to understand our emotions and opinions faster and in a better way.

The answer might be a hope