Specify the format parameter to change the output format. To see the non-HTML representation of the JSON format, set format=json.
It would be nice if format=json were a hyperlink. For example, in this case it would link to https://en.wikipedia.org/w/api.php?action=query&prop=extracts&titles=All_I_See_%28A%2B_song%29&explaintext=1&exsentences=10&format=json.