wagtail/client/scss/components/forms/_error-message.scss

@use 'sass:map';

.error-message {
  border: 1px solid transparent; // ensure visible separation in Windows High Contrast mode
  margin: 0.5em 0 0;
  font-size: 1em;
  font-weight: bold;
  color: theme('colors.text-error');

  &::before {
    content: '';
    display: inline-block;
    width: 1em;
    height: 1em;
    vertical-align: -10%;
    mask-image: url('#{$images-root}icons/warning.svg');
    background-color: currentColor;
  }
}