... And all the other stuff
Java is just about unusable on the Mac, it's not too hot with Windows NT either. Only Windows 95/98 and UNIX machines have a reasonably reliable Java. I have had to disable Java on both my Mac and NT Server through problems with slowness and crashing.
Plug-ins are platform specific. Some of the more common ones are available for Mac and PC but many others are not. Remember, using any plug-in whatsoever is restricting your potential audience very considerably and choosing to design a Web site that depends on the reader having a specific browser or plug-in is a decision that can't be taken lightly.
Cascading Style Sheets, the technology that has the greatest potential to improve Web page typography is severely hampered because of the differences in implementation between Netscape and MsIE.
Here is an excellent new resource about Cascading Style Sheets http://www.westciv.com/style_master/house/ which gives all the gory details.
In a perfect world, all browsers on all platforms would interpret HTML code in the same, consistent way and Web page design would be easy. In reality, ignorance is bliss and the more you know about Web design, the more difficult it becomes.
The moral to all this is that the simpler your design is, the less problems you are likely to have.