Marketo has validation rules turned on for some fields, for example Email should be a valid Email Address, clicking on and off the field will activate the error message, other field types have the same sort of features, see here - Custom Field Type Glossary - Marketo Docs - Product Docs
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Yes, Min, you can do this.
Using CSS, hide the .mktoError and related styles by default. Then, in a custom onValidate function, restore the visibility (an easy way is to add a class to the <FORM> that overrides the previous styles). The onValidate does not run when entering or leaving the field, but only (by default) when the submit button is pressed.