By default the browser specific error texts are used, like missing required fields or pattern are not matching. With Custom Error Texts can custom error texts displayed.
To create a new Custom Error Text go to https://flyingforms.io/projects/[YOUR_PROJECT]/Forms/ErrorTexts/Index.
Only basic styling like coloring i provided. To style the error texts, a custom css is neccessary.
Custom Error Texts are objects that needs to be selected in the form designer. Within the left navigation in form designer is a section named Error Texts. There can the definition be chosen.