Something that makes a website great is when it is compatible in multiple browsers regardless of version. Technically this is referred to as cross-browser compatibility.
Wikipedia defines cross-browser as:
The ability for a website, web application, HTML construct or client-side script to support all the web browsers.
Now, you may be confused on what is the difference between cross-browser and multi-browser.
Multi-browser means a website will work in several web browsers like Safari, Chrome, Firefox, Internet Explorer. While cross-browser means a website works in any browser, and any version of the browser, being used.