Remember that CSS uses a “cascade” meaning that rules that are defined below another rule will take precedence. For example if I defined some rules like this:
p {
font-size: 16px;
font-size: 18px;
}
In the above example the font size of p tags will be 18px because that’s the line that is lower in the cascade. It overrides the font-size: 16px.
However if that last line were to not work for some reason (like if I were using a CSS feature that the browser didn’t support such as in the case of IE). then the line font-size: 16px would be the “fallback”.