cannot be defined on per input basis. You can create a custom rule that matches a single input only and provide a message of your choice for it, as shown in this code snippet
If you need to change the styling of the input while it is in the checking state, consider adding a custom CSS class to it when you enter the availability.check
function (from the example in the blog post). Using this class, you can overwrite the default error style during the AJAX call only. In the AJAX request complete
callback, you can remove this class from the input element, so the normal valid/invalid styles can kick in.
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