I recently found this series of videos on the history of mahou shoujo anime by SourcererZZ. Somehow, he’s managed to dredge up clips from early shows, including some that are quite hard to find. He seems to know his subject very well. Depending on what he gets into, some of his later videos might be NSFW, but this first one is a pretty good overview of the evolution of the genre from its origins with Sally the Witch to the early 1970s with Cutie Honey (that last getting a little racy toward the end of the video).
He’s a little hard to understand and rather quiet, but if you turn on the closed captions, he’s added accurate subtitles.