The a cappella guitar solo in Heartbreaker — played with no backing track — is one of the earliest and most influential examples of unaccompanied electric guitar in rock. Page plays a cascading series of hammer-ons and pull-offs that predates Eddie Van Halen's tapping technique and demonstrates that Page was already exploring the instrument's solo capabilities years before the technique became mainstream.

Reference:

Wikipedia: Heartbreaker

https://en.wikipedia.org/wiki/Heartbreaker_(Led_Zeppelin_song)