JavaScript Browser check:

function bName() {
  if (navigator.appName == "Microsoft Internet Explorer")
    return 1;
  if (navigator.appName == "Netscape")
    return 2;
  return 0;
}

function bVer() {
  return parseFloat(navigator.appVersion)
}

<script language="JavaScript">
<!--
var nameCode = bName();
var versionCode = bVer();
if ((nameCode != 0) && (versionCode >= 4)) {
  // stmts
}