The most ideal solution would be to support any browser that your users use.
The next best solution is to tell your visitors if their browser is not supported, and how to upgrade or switch browser.
The worst solution is to just let the website break in mysterious ways on unsupported browsers.
People on old browsers don't need to experience websites that ...
In the HTML, label the email field "username", both name and ID attributes. Use "Email" as the label that the user sees. They will be setting a login that is an email.
Don't capture a separate username, as that might be confusing ("Which do I log in with?") Use Display Name as a handle that will be used throughout the site.