Inside your dropzone HTML element (in most cases that's a <form> element), you can define an additional element with the class fallback . When (or in this case: "if") Dropzone initializes, it will simply remove this element.
Here is an example of what this might look like:
So in case the browser doesn't support JavaSript, the user will simply see the fallback div with the file input element, and everything will work normally but with no drag and drop support.
If the browser is supported however, this element will be removed, and Dropzone will add a notice that it accepts dropped files and handle them.