They
did write their own music.
Most of what they stole was just lyrics (not their claim to fame), and pretty much
all of it was just from their first two albums, after III they were doing more original compositions.
They did a few uncredited covers (Babe I'm Gonna Leave You, Dazed and Confused) which was wrong, but their versions are greatly different from the originals anyway, they made them their own.
There are only two songs from them musically that I would say are direct ripoffs, Black Mountain Side (originally by Bert Jansch) and Hats Off To Roy Harper, the later of which they credited as a traditional piece from the start so who cares?
Besides, both of those tracks are good competitors with Hot Dog for the title of
most insignificant song in Led Zeppelins entire catelog.
Its amazing, the desperate measures you guys take to discredit a band you hate. Absolutely pathetic.