Tina,
i was getting the same error. Deepak is right. In my case, it was not the Html tags, but an extra comma
<APPLET CODE="HelloAgainApplet.class" WIDTH=250, HEIGHT=70>
once i took the comma after, "WIDTH=250" out
<APPLET CODE="HelloAgainApplet.class" WIDTH=250 HEIGHT=70>
the 'appletviewer' started working alright.
neeraj singh