Here's a table of Browsers versus the JavaScript versions they claim to support:

                IE   IE   IE   IE  IE  IE  IE  IE
                PC   Mac  PC   PC  Mac Mac PC  PC    Netscape   Mozilla   Opera
                4.01 4.01 4.01 5.0 5.0 5.1 5.5 6     4.7  6.0   0.9-1.0   6.03

JavaScript      X    X    X    X   X   X   X   X     X    X        X      X
JavaScript1.0   X    X    X    X   X   X   X   X     X    X        X      X
JavaScript1.1   X    X    X    X   X   X   X   X     X    X        X      X
JavaScript1.2   X    X    X    X   X   X   X   X     X    X        X      X
JavaScript1.3             X    X   X   X   X   X     X    X        X      X
JavaScript1.4                      X   X                  X        X      X
JavaScript1.5                                             X        X
Jscript         X    X    X    X   X   X   X   X
EcmaScript                X    X           X   X                          X
exceptions      no   no   yes  yes yes yes yes yes   no   yes     yes     yes
supported?

An "X" in row "Yyyyy" means code under <script language="Yyyyy"> will run
in the browser for that column.  For example:

     IE6/PC
     ------
     <script language="JavaScript 1.3">
          // runs
     </script>

     Netscape 6
     ----------
     <script language="EcmaScript">
          // ignored
     </script>

(Yes, there are two radically different browsers both claiming to be IE4.01,
one claims to support JavaScript 1.3 and EcmaScript, and exceptions work
perfectly, and the other not.)

See the Error section for a trick on how to include code with exceptions.