What does the 'action' attribute of a form element specify?

The 'action' attribute of a form element is crucial because it designates the URL where the form data will be sent when the user submits the form. This attribute is essential for defining the endpoint that will process the data provided in the form fields. When a form is submitted, the browser will gather all the input values and send them to the specified URL to be processed by the server or a specified script.

The other attributes mentioned in the context of the question refer to different aspects of form handling. For instance, the method used to submit the form pertains to how the data is sent (such as GET or POST), while the types of inputs allowed relate to input validation and restrictions. Lastly, appearance concerns the visual styling of the form elements but does not impact functionality or data submission. Thus, the 'action' attribute is specifically tied to the destination for the submitted data, making it a fundamental aspect of form submission in web development.