Very doable - but not a native surface concern.
Phoenix does a bit of magic to hide errors on fields the user has not yet interacted with.
This ‘magic’ is done with these classes when using ErrorTag:
.phx-no-feedback .invalid-feedback {
display: none;
}
Obviously display:none; would not work on a border - and this will have to be modified achieve applying an error border to a field.
Think you would apply a default border if .phx-no-feedback is present, and when just .invalid-feedback is present on the field, apply the error border or similar…
Could also use more intelligent CSS selectors like if .invalid-feedback is present on a parent node - to apply certain styles…
