Question

What does the :not(form) part of a CSS selector for angular2 form elements?

Context

The current version of the Angular2 tutorial about forms makes use of two-way data binding, change tracking and validation to add CSS classes to form elements.

Their example form looks like this

And their example code to achive the red bar looks like this

.ng-invalid:not(form)  {
   border-left: 5px solid #a94442; /* red */
}