When I was starting to learn Bootstrap for my GitHub App, I came across this CSS Framework that will simplify my medium-sized project.
Having read only the introduction, I realized this is really great, just by adding it to your HTML, you can start using it.
So let's have a basic Hello World
<!DOCTYPE html>
<html>
<title>W3.CSS</title>
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://www.w3schools.com/w3css/4/w3.css">
<body>
<div class="w3-container w3-teal">
<h1>Hello World</h1>
</div>
</body>
</html>
you can use JSFiddle to run the code to see the result right away https://jsfiddle.net/
or W3schools https://www.w3schools.com/tryit/tryit.asp?filename=tryhtml_hello
this is the sample screenshot,
as you can see just by adding the line
<link rel="stylesheet" href="https://www.w3schools.com/w3css/4/w3.css">
you are already set and start your project right away.