Hi!
The current validation looks like this:

Setting the border color instead would look much better:

I hope this will be included in a future release. Thanks.
The following styles solve the problem:
.mdc-text-field__input.invalid {
border-bottom: none !important;
}
.invalid + .mdc-notched-outline > .mdc-notched-outline__leading, .invalid + .mdc-notched-outline > .mdc-notched-outline__notch, .invalid + .mdc-notched-outline > .mdc-notched-outline__trailing {
border-color: red !important;
}
@kristof12345 thanks for finding a solution. Do you want to submit a PR on develop for this fix?
@enkodellc Ok. I tried to create a new branch, but I鈥檓 not allowed to push it. (This would be my first contribution to a public repo, so I might be doing something wrong. )
Another question: what do you use to compress the css to a single line?
@kristof12345 you do not have rights to create a new branch or push. You will need to Fork the repo on your account. Then submit a PR using your Fork, don't forget to change the branch to develop.
Once you submit a PR I can revew and merge it into Develop. I don't have rights to merge to master.
This will help: https://opensource.com/article/19/7/create-pull-request-github
I know it is not super straight forward but it is a skill you will use in the future as you contribute more and it is an asset in your developer arsenal :)
Fix is in Develop Branch.