every browser has a default stylesheet (aka “user agent stylesheet”)

a small sample from the Firefox default stylesheet:

h1 {
   font-size: 2em; 
   font-weight: bold;
}

different browsers have different defaults

Illustration of a smiling stick figure with curly hair.

person: buttons & forms have some of the biggest differences

you can read the default stylesheet

Firefox’s default stylesheets are at:
resource://gre-resources/

every property also has a default “initial value”

the initial value (defined in the spec) is what’s used if no stylesheet has set anything. For example, background-color’s initial value is transparent

a CSS property can be set in 5 ways

(listed from lowest priority to highest priority)

1. the initial value
2. the browser’s default stylesheet
3. the website’s stylesheets and user stylesheets
4. inline styles set with HTML/JS