Hiding a textfield on an external website form - html

Question

Is it possible to hide a text field on the form of a website. The text field is currently not hidden and I've heard that I could be able to hide it from view by overlaying an external style sheet.
EDIT: The website isn't mine and I have no control of the source etc, I need to be able to do this without touching their code. The reason we need to be able to do this is to send a credit card number to this field from a session variable (obtained by DTMF tones and an IVR) without the person sat in front of the computer being able to see what's been inputted.
I don't want to hide it from EVERYONE, just the people who work for us and access it through our PC's.
Is this possible?
Thanks,

Answer 1

Yes. Add the following css class:
display:none;

Answer 2

yes you can
CSS Approch
use css display:none;
Or use javascript
var child = document.getElementById(childDiv);
var parent = document.getElementById(parentDiv);
parent.removeChild(child);

Answer 3

Depending on what you are allowed to do, and whether these machines are only used for that site, you can create a page with an iframe holding the site you want and then overlay what you need.
It might be tricky if the page is scrollable, or the user can alter the size of text etc (cause reflows to the content)
<iframe src="http://url.to.actual.page"></iframe>
<div class="distractor">Position me on top of the iframe to hide some content</div>
and
iframe{
width:600px; /* as needed */
height:300px;
overflow:hidden;
z-index:1;
position:relative;
}
.distractor{
position:absolute;
background-color:red; /* color it as the background of the page */
top:80px; /* position it where you need it */
left:100px;
width:100px; /* size it as you need it */
height:100px;
z-index:1000;
}
demo at http://jsfiddle.net/gaby/GemC2/1/
Not a great solution, but might give you some ideas..
Another solution, if you are allowed to use plugins to browsers is to set Greasemonkey, and install a custom script that will hide the input you need.
Or Stylish and set a custom CSS style to override the page's style.