modern.IE is a site released by Microsoft that provides a set of tools that helps your website support modern and older versions of Internet Explorer. One can scan a webpage to find common browser compatibility problems, and it allows 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 that can bring visual testing of your site directly on your browser and locally managed virtualization) can help provide quick access to various browsers.
There is a special offer on Browser Stack; one can get three months of free testing and provide add-ons for Chrome and Firefox, which integrates BrowserStack testing into your browser.
So go and test your website live on modern.IE site, figure out the issue, and download the available Virtual tools to resolve the problems.