You will have to decide what screen sizes you will support. In the case of Lukas , a high tech IT ERP Boeing Subcontractor, the size of the web pages made in the spring of 2000 support a twelve inch screen. That should tell you that it is smart to realize that every visitor you turn away due to technical incompatibility means revenues lost. Each decision you make could exclude some of your potential customers from your web pages. Unless you site does something truly amazing, computer users will leave when confronted with error messages or an unresponsive system.
People find plug-ins a nuisance.
A two megabyte plug-in requires ten minutes to download, after which a user will have to install the software and reboot the computer. Only a small faction of your customers will ever put up with this sort of consumption of their time. Remember your decision to develop a web-site is motivated by a desire to get the extra cash flow it promises. You have to balance the design constraints of 3.4, 4.0, 5.0 browser versions with windows, Max and Linux operating systems, as well as with different plug-in. For maximum compatibility, your site must be tested with a many different combinations of these characteristics as possible.