I've lightened the colours, and float and z-index now occur on the same line - so there is less likelihood of a scrollbar appearing.
Code:
javascript:(function(){var d=document,useMine=true,prevEl,info;function addHandler(orig,mine){return function(e){if(useMine){mine(e);}else if(orig){orig(e);}};}function GS(el,sRule){var result='';if(d.defaultView&&d.defaultView.getComputedStyle){result=d.defaultView.getComputedStyle(el,'').getPropertyValue(sRule);}else if(el.currentStyle){sRule=sRule.replace(/\-(\w)/g,function (strMatch,p1){return p1.toUpperCase();});result=el.currentStyle[sRule];}else{result='n/a';}return sRule+': '+result;}function myover(e){var el=e?e.target:window.event.srcElement;el.style.outline='1px solid red';var sInfo=el.nodeName;if(el.id)sInfo+=" ID: "+el.id;sInfo+="\n";if(el.className)sInfo+="Class: "+el.className+"\n";sInfo+=GS(el,'width')+" "+GS(el,'height');sInfo+="\n"+GS(el,'padding');sInfo+="\n"+GS(el,'border');sInfo+="\n"+GS(el,'margin');sInfo+="\n"+GS(el,'display');sInfo+="\n"+GS(el,'position');sInfo+="\n"+GS(el,'font');sInfo+="\n"+GS(el,'float');sInfo+=" "+GS(el,'z-index');info.value=sInfo;prevEl=el;}function myout(e){var el=e?e.target:window.event.srcElement;if(!el.keepOl)el.style.outline='';}function mymove(e){var evt=e||window.event;var el=evt.target||evt.srcElement;info.style.left=parseInt(evt.pageX)+20+"px";info.style.top=parseInt(evt.pageY)+10+"px";}function myclick(e){var evt=e||window.event;var el=evt.target||evt.srcElement;info.style.left=parseInt(evt.pageX)+4+"px";info.style.top=parseInt(evt.pageY)+4+"px";el.keepOl=true;createBox();if(el.href){var temp=el.href;el.href="#";window.setTimeout(function (){el.href=temp;},20);}evt.preventDefault;return false;}function mydown(e){var evt=e||window.event;if(evt.keyCode==27){if(!prevEl.keepOl)prevEl.style.outline='';useMine=false;info.parentNode.removeChild(info);}}function createBox(){info=d.createElement('textarea');info.style.position="absolute";info.style.width="250px";info.style.height="170px";info.style.zIndex="999";info.style.fontSize="11px";info.style.color="black";info.style.backgroundColor="lightyellow";info.style.paddingLeft="3px";d.body.appendChild(info);}d.onmouseover=addHandler(d.onmouseover,myover);d.onmouseout=addHandler(d.onmouseout,myout);d.onmousemove=addHandler(d.onmousemove,mymove);d.onclick=addHandler(d.onclick,myclick);d.onkeydown=addHandler(d.onkeydown,mydown);createBox();})()