About

A user agent delivers a default Style Sheet.

Every browser provides a default set of styles also known as user agent styles.

Operation

Normalize

The provided style simply override the browser defaults to get the same environment on all browser

Example: https://necolas.github.io/normalize.css/

Disable

You can disable the default appearance of object such as radio with the appearance property 

input[type="radio"] {
  appearance: none;
}