A ruleset consists of a selector (or a group of selectors) paired with a declaration block.
A declaration block {} contains one or more declarations.
A declaration Property: Value; sets a value for a property.
Declarations must end with a semicolon, except for the last one. Adding a semicolon to it is recommended, though.
* {
box-sizing: border-box;
}
h1,
h2 {
color: black;
font-weight: bold;
font-size: 3rem;
}