Every website performs differently.
For Web designers and developers, cross-browser compatibility has been a long term issue. Each browser implements JavaScript, HTML, CSS etc. somewhat differently and this can result in a myriad of issues for the user; from barely noticeable visual differences to pages that break across browsers..
Although the W3C has created standards for the various formats in an attempt to ensure content is displayed consistently across all browsers, adhering to them is recommended only, not compulsory, and as a result many sites are still not cross-browser compatible. While this may not be a major concern for everyone, when it comes to business it's essential.