I think they are suggesting using something like this, which does work to target certain attributes:

input[value="5"] { width: 20%; }

This could then be combined with the sibling selector to target the other one

input[value="5"] + input[value="11"] { width: 80%; }

Though I am unsure how to make it entirely workable (you probably need a simple event listener to set the DOM node value to the dynamic form value). Interesting exercise if nothing else, and I think it could be doable to reach a working solution.