In the attached test case, Firefox gives none/none/changed/changed and IE gives none/none/auto/auto, indicating that they at least returned undefined or a default value, respectively. Safari, however, gives none/none/changed/changeme and displays "Undefined value" and "Null value" in the error console, preventing further JavaScript execution.

An undefined return value is fine, but it probably shouldn't throw an exception. A common idiom in the UI classes is 'parseInt($(el).css(prop)) || 0', assigning a default value if the property doesn't exist. This behavior causes the whole widget to break instead of functioning with the defaults (I first noticed it with UI Slider).