DOCTYPE, javascript and Firefox

I came across an odd thing with DOCTYPE, javascript and Firefox 3 that has me stumped. Not exactly a php issue but hoping someone else on the list has seen this before. (At least the script is php, hope that counts) I have the following code in my header script: echo "<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\" \"http://www.w3.org/TR/html4/loose.dtd\">\n"; Elsewhere I have (simplified): echo "<input type='radio' name='autostart' value='0'" ."onclick=\"autostartlabel.className='disable';" ."article.startdate.disabled=true;\" />No" ."<input type='radio' name='autostart' value='1'" ."onclick=\"autostartlabel.className='normal';" ."article.startdate.disabled=false;\" />"Yes"; ... echo "<label id='autostartlabel'>Startdate" ."<input type='text' name='startdate' id='startdate' disabled=true />" ."</label>"; It's simply 2 radio buttons and if "yes" is clicked related form fields are enabled, if "no" is clicked they're disabled. This code works fine in IE, Opera and Chrome, but gives a javascript error in FF when I click the radio button: "autostartlabel is not defined". However if I comment out the DOCTYPE line in the header it works fine in all browsers including FF. (Took ages to narrow it down to the DOCTYPE.) Does anyone know why, and how to fix it? Cheers Arno