It's an ever-changing landscape.

UNIX
HTML 1.0, HTML 2.0, HTML 3.0, and HTML X.0
Perl
CGI scripting
Java scripting
PGP
PDF

Plus an ever growing list of software tools related to databases, secure commerce, data compression, image conversion, data capture, scanning, file transfers, animation, editing and publishing, as well as site management. And sound and video and 3D and VRML.

Viewed on:

Netscape 1.0, 2.0, 3.0, 3.01, and 4.0
Explorer number, number, number, number
AOL... yes, AOL
Images On
Images Off
Mac
PC
modems at 14.4k, 28.8k, 33.4k, 56k
ISDN
Network

Whew!