Or that Happy Days was itself a spinoff, having originated as an episode of the comedy anthology series Love American Style.
Well, that's not entirely accurate. It originated as an independent pilot called New Family in Town, but when the pilot didn't sell to any of the networks, Garry Marshall sold it to L:AS producer Aaron Spelling, who turned it into a segment of his show. A lot of L:AS segments were cribbed from rejected pilot scripts, though this was a case where the pilot was actually shot before it ever became part of L:AS. So it's not really a spinoff of L:AS, it's a show whose rejected pilot happened to get shown on L:AS, and that then got its rejection reversed after American Graffiti and Grease were hits.