[Affected versions]: Nightly 63.0a1 [Affected platforms]: Platforms: Windows 10 x 64, Mac OS X 10.12 and Ubuntu 17.04 x64. [Preconditions]: 1. Set the pref dom.payments.request.enabled to "true"; 2. Set the pref dom.payments.loglevel to “Debug” to increase the verbosity of console messages. 3. Make sure you have at least one Shipping Address and one Shipping Option saved in your browser (Saved Cards). [Steps to reproduce]: 1. Go to "https://rsolomakhin.github.io/pr/us/" page and click on "Buy". 2. From "Your Payment" page, click add a new shipping address. 3. Fill some of the mandatory field and leave some of them empty and click on "Save". [Expected result]: Submitting shipping address form shouldn't be allowed without filling all the mandatory fields [Actual result]: The "Save" button should be grayed out or an error should be displayed when user want to submit a form without filling all the mandatory fields.