Sites are designed with standards in mind. This allows for consistent results across all major browsers. There are times when a specific web browser does not support a standard. In these cases, the goal is to aim for the best alternative possible for that browser. If a browser has less than 5% market share, then no additional efforts will be put towards that browser.
For example, CSS3 gradient only works in IE 10 or above. IE 9 has been less than 1% market share for all of 2016. Therefore a background will just have a solid color in IE9, but display as a gradient in current browsers.