When you build your own custom form and add the JS-validation to it, the validate.js demands that you use the html5 “required”-tag.
This tag prevents you from using your own validation messages in regular browsers AND does not work in Safari.
Validation should be possible without using the required-tag.
See: https://www.thenewsletterplugin.com/forums/topic/no-built-in-validation#post-39231