When implementing 3D Secure with Braintree for a web page, the bank's verification page is passed into the JavaScript function as an iframe element to embed into the web page. Unfortunately, the bank pages have fixed 400px width, therefore they don't fully fit on narrower mobile screens.
Braintree's Hosted Fields offering is a great compromise between payment form customization and PCI SAQ A compliance requirements. Although it is not immediately evident from the documentation, the payment form can also easily be extended with custom data fields that your business might require. This post demonstrates how to do it in an ASP.NET Core application.