ISDM version 1 was built specifically for Netscape 1.1. It uses multiple body tags, which are no longer allowed by Netscape 2. At the ISDM site site you can see a version that has been adapted by ISDM to the new regime.
Note that I do not work with ISDM anymore. I can not be accounted for any design irregularities in this version.

Oh, yes! I still have here the very first ISDM home pages designed by Zupergraphyx!, one of my first HTML3 designs. They were only on line for a few weeks in May 1995, but they still look incredibly fresh, I think.
(Only the "Hyperphernalia Corner" and the "Favourite Places" links work. Don't bother clicking on the others.)

The ISDM site was in The Fridge on June 2nd, 1995 and it got four "UP"-stars in its ART section.
The ISDM site was The Web Seeress Vision of June 14, 1995.
Unit Circle gave the ISDM site a star quotation.
The ISDM site was in Jeff's Junky Junk.
The ISDM site was Arts site of the Day on August 17th, 1995.