1. 'null' is empty or not an object: Try adding a defer=true attribute to it, <br>that is: <script type="text/javascript" defer=true> <br><strong>2. document.getElementById is null</strong> <br>Reason: var titab=document.getElementById("titletab"); was not retrieved Object <br> Solution: <br>1. Add alert(titab); debugging statement below the error statement to see if a message box pops up <br>2. If it does not pop up, check whether it exists in the HTML code The <table> tag of "id=titletab" (be careful, you can’t make a mistake with a single letter) <br>3. If the tag exists, consider the location of the <script> code segment, and remember to include code similar to getElement The <script> code snippet must appear after the HTML Element. That means you put the <script> code snippet after the <html> tag and try. </p>