A Netscape web browser
A MSIE browser
Several other new web browsers
A server CGI environment
A desktop application environment
A UNIX shell
A WebTV set top box
A Liberate TV Navigator set top box
A WAP/WScript mobile phone
An Adobe PDF file reader
An embedded web browser built-into consumer products
Each of these has certain advantages and constraints. Most offer special facilities native and unique to that hosting environment.
In general, you should be able to determine which of these environments you are operating in. However, there is no standardized way to detect this at present.
There may be range limits on values in certain environments and certainly there will be 'bugs' in the implementations that are platform specific. It is also very likely that functionality will be more or less incomplete in some environments - mostly depending on the maturity of the implementation.