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I've looked at Firefox's, but it's not normal CSS that you'd be looking at. One of Firefox's CSS file is located at: /Applications/Firefox.app/Contents/MacOS/res/forms.css. There's multiple CSS files though.

Here's a sample piece from the file,
Code:
input {
  -moz-appearance: textfield;
  /* The sum of border-top, border-bottom, padding-top, padding-bottom
     must be the same here, for buttons, and for <select> (including its
     internal padding magic) */
  padding: 1px 0 1px 0;
  border: 2px inset ThreeDFace;
  background-color: -moz-Field;
  color: -moz-FieldText;
  font: -moz-field;
  text-rendering: optimizeLegibility;
  line-height: normal !important;
  text-align: start;
  text-transform: none;
  word-spacing: normal;
  letter-spacing: normal;
  cursor: text;
  -moz-binding: url("chrome://global/content/platformHTMLBindings.xml#inputFields");
  text-indent: 0;
  -moz-user-select: text;
}
Not sure how to get at Safari's CSS files.
 
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