modern.IE is a site released by Microsoft which provides a set of tools that helps your website to support modern and older versions of Internet Explorer. Where one can scan a webpage to find common browser compatibility problems and helps you to resolve them by stating related solutions.
modern.IE also offers various virtual testing tools by which you can test your site for compatibility from any browser from a Mac or Windows PC. Its hosted (a cloud-based virtualization service which can bring visual testing of your site directly on your browser and locally managed virtualization can help to provide quick access to various browsers.
So go and test your website live on modern.IE site, figure out the issue, and download the available Virtual tools to resolve the issues.