Visually indicate form inside IFrame is submitting, but without javascript

I have an iframe in a page that is sandboxed to disallow JavaScript.

When the user clicks the form's submit button there is no visual indicator to the user that something is happening.

Is there a way I can use CSS to show something is happening without requiring javascript to be enabled?

For example 1: An animation on the button that starts when the user clicks it and runs for 5 seconds. 2: A pseudo element on <form> that is only present when the form is being submitted. 3: Some way of styling the IFrame differently when it is attempting to navigate.

Anything at all?