как-то так:
var
msg = '<HTML>htmltext1<\/HTML><CSS>csstext<\/CSS><SCRIPT>scripttext<\/SCRIPT>',
HtmlContent = (/<HTML>(.*)<\/HTML>/.exec( msg ))[1],
CssContent = (/<CSS>(.*)<\/CSS>/.exec( msg ))[1],
ScriptContent = (/<SCRIPT>(.*)<\/SCRIPT>/.exec( msg ))[1];
alert ( "HTML:\n" + HtmlContent );
alert ( "CSS:\n" + CssContent );
alert ( "SCRIPT:\n" + ScriptContent );