Source: 📖 Test Driven Development with Python ch14 p252
Django forms automatically add HTML attributes to fields. One of these attributes is the required attribute for required fields. In doing so, the browser can validate user input client-side and display an error message if the user attempts to send faulty data.
We can test for the presence of these errors by finding the CSS pseudo-selectors :valid and :invlaid. For example, if we have a text field, the Django form will automatically give it the ID name id_text when it creates the HTML for the form. We can find CSS elements with the syntax #{id_name}:valid or #{id_name}:invalid—in this case, that would be id_text:valid and id_text:invalid.