Great @owen.campbell! I also did something similar using roles in the last couple of days.
My CSS entry:
.anvil-role-glowing-red-border {
outline: none;
border-color: red;
box-shadow: 0 0 10px red;
}
which gives this effect:
But I then implemented a load of logic that checks the fields on change etc. and applies / removes the role on the component accordingly.
So I’m now going to take your approach using the placeholder bits and merge that into mine (and delete some code yay!)