Keep your pages simple. Less is more. Don't use frames unless you really know what you're doing. There are few things more aggravating than having your computer crash because a web designer couldn't use frames correction. Sounds and bits of Java should also be used with caution.
Most people find flashing titles and other blinking things really annoying.
All browsers are not created equal. If your HTML works and looks OK on all three, you've done well. Colors will look different on every monitor. Don't put spaces in the names of any HTML or image files. Though this page deals most specifically with using Claris HomePage, it's generally a good idea to learn at least a little about what raw HTML looks like - it will make troubleshooting a lot easier.