CSS to identify empty form fields

I am looking to identify which of my form fields are empty when the form renders by putting a red border around the field input box. I have attempted css for .kn-input :placeholder-shown or input:empty but neither of these work. Knack support says I need a combination of JQuery and CSS to loop through the data - is this true or is there some obvious way of going about this without JQuery (I am not familiar with Javascript).

Any help would be appreciated.