We highly recommend reading the Using The Web SDK guide. The guide includes a basic example and instructions for using Gigya's Web SDK methods.
Please see here for supported browsers.
If the Web SDK makes an API request that results in an UNVERIFIED_USER (403013) response, it will, internally, make another API request to accounts.verifyLogin (an internal method) and then repeat the original request.
This can happen, for instance, when a Site Admin publishes a new consent version and changes the minimum required version.