As this great article explains:
In the beginning, there were three ways to specify colors: in hexadecimal format, using either three or six characters, named colors such as
rgb(). CSS3 has since added several new color formats including
hsla(). While these new color formats bring a remarkable amount of flexibility to how developers define colors and the relationships between them, it can also leave older browsers looking worse than expected.
The problem is that CSS parsers in browsers will skip a property whose name or value is not understood. Older browsers such as Internet Explorer 8 and earlier, do not understand
hsla(), and as a result will drop any declarations containing them.