When entering text in a field, the associated label disappears: How to find guilty JavaScript

Generally speaking, I'm a decent programmer, (C++, Java, Ruby, Python) but I at the very starting poin开发者_如何学JAVAt of learning Javascript. I'm debugging inherited code (so I'm unfamiliar with it) and the current problem is that whenever I click a text field, the associated label goes away.

How can I tell what JS is causing this?

(My browser of choice is Chrome.)

Install the Firebug extension for Firefox. In the HTML tab there's a button "break on mutate" which will act as a breakpoint that triggers as soon as the DOM is modified. This should help you finding out what script is causing your issue.