TYPO3 Core

from the fle form.css:

#fake-form .formwizard-element button.t3-icon-edit-delete {
    background-image: url('../../../../../core/Resources/Public/Icons/T3Icons/actions/actions-delete.svg');
}

the relative path of the icon is wrong, and it ends up as http://yourhost/typo3/core/Resources/Public/Icons/T3Icons/actions/actions-delete.svg instead of

http://yourhost/typo3/sysext/core/Resources/Public/Icons/T3Icons/actions/actions-delete.svg

solution: there is an excessive ../ simply remove it.

#fake-form .formwizard-element button.t3-icon-edit-delete {
    background-image: url('../../../../core/Resources/Public/Icons/T3Icons/actions/actions-delete.svg');
}