There are scalable units, like vw and vh etc that are 1% of the browser's viewport, vw is like innerWidth / 100. So something like:
h1 {
font-size: 10vw;
}This means 10vw is going to be 50px when the browser is 500px wide, and 100px when the browser is 1000px wide for example. Using scalable units by themselves can sometimes lead to things being too-big or too-small, so some people use them with @media queries to 'switch them off' to some other unit at a certain size.