/* Wrapper function - split search text into separate phrases and words */
function highlightSearchTerms(searchText)
{
  if (!document.body || typeof(document.body.innerHTML) == "undefined") return false;
  if ( typeof(searchText) == "undefined") {
    searchText = getParameter("srch");
    // alert("Search String = [" + searchText + "]");
  }
  var bodyText = document.body.innerHTML;
  var searchArray = searchText.split(" ");
  for (var i = 0; i < searchArray.length; i++) {
    // alert("Highlight word = [" + searchArray[i] + "]");
    bodyText = doHighlight(bodyText, searchArray[i]);
  }
  document.body.innerHTML = bodyText;
  return true;
}

/* Highlights a text string by adding HTML tags before and after the search term. */
function doHighlight(bodyText, searchTerm)
{
  const highlightStartTag = "<font style='background-color:yellow;'>";
  const highlightEndTag = "</font>";
  var newText = "";
  var i = -1;
  var lcSearchTerm = searchTerm.toLowerCase();
  var lcBodyText = bodyText.toLowerCase();

  while (bodyText.length > 0) {
    i = lcBodyText.indexOf(lcSearchTerm, i+1);
    if (i < 0) {
      newText += bodyText;
      bodyText = "";
    } else {
      // skip anything inside an HTML tag
      if (bodyText.lastIndexOf(">", i) >= bodyText.lastIndexOf("<", i)) {
        // skip anything inside a <script> block
        if (lcBodyText.lastIndexOf("/script>", i) >= lcBodyText.lastIndexOf("<script", i)) {
          newText += bodyText.substring(0, i) + highlightStartTag + bodyText.substr(i, searchTerm.length) + highlightEndTag;
          bodyText = bodyText.substr(i + searchTerm.length);
          lcBodyText = bodyText.toLowerCase();
          i = -1;
        }
      }
    }
  }
  return newText;
}

/* Get URL parameter */
function getParameter(paramName) {
  var currentUrl = window.location.search;
  var strBegin = currentUrl.indexOf(paramName) + (paramName.length+1);
  var strEnd = currentUrl.indexOf("&",strBegin);
  if (strEnd==-1) strEnd = currentUrl.length;
  var paramValue = unescape( currentUrl.substring(strBegin,strEnd) );
  while (paramValue.indexOf("+") > 1 ) paramValue = paramValue.replace("+", " ");
  return paramValue;
}

<!--
  /* This belongs in the global stylesheet. */
  .highlight { color: black; background-color: yellow; }
-->

<span class="highlight">keyword</span>

function doHighlight(bodyText, searchTerm, highlightStartTag, highlightEndTag)
{
        // find all occurences of the search term in the given text,
        // and add some "highlight" tags to them (we're not using a
        // regular expression search, because we want to filter out
        // matches that occur within HTML tags and script blocks, so
        // we have to do a little extra validation)
        var newText = "";
        var i = 0, j = 0;
        var before_c, before_c;
        var lcSearchTerm = searchTerm.toLowerCase();
        var lcBodyText = bodyText.toLowerCase();

        while ((i = lcBodyText.indexOf(lcSearchTerm, i)) > 0) {
                before_c = lcBodyText.charCodeAt(i-1);
                after_c  = lcBodyText.charCodeAt(i+searchTerm.length);
                if (lcBodyText.lastIndexOf(">", i) >= lcBodyText.lastIndexOf("<", i)) {
                        if (lcBodyText.lastIndexOf("/script>", i) >= lcBodyText.lastIndexOf("<script", i)) {
                                if ( (before_c < 65 || before_c > 122) && (after_c < 65 || after_c > 122) ) {
                                        newText += bodyText.substring(j, i) + highlightStartTag + bodyText.sub
str(i, searchTerm.length) + highlightEndTag;
                                        i += searchTerm.length;
                                        j = i;
                                        continue;
                                }
                        }
                }
                i++;
        }
        newText += bodyText.substring(j, bodyText.length);

        return newText;
}