demo/demo5.html

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>My first three.js app</title>
		<style>
			body { margin: 0; }
			canvas { width: 100%; height: 100% }
		</style>
</head>
<body>

</body>
<script src="https://cdn.bootcss.com/three.js/91/three.js"></script>
<script>
var renderer = new THREE.WebGLRenderer();
renderer.setSize( window.innerWidth, window.innerHeight );
document.body.appendChild( renderer.domElement );

var camera = new THREE.PerspectiveCamera( 45, window.innerWidth / window.innerHeight, 1, 500 );
camera.position.set( 0, 0, 100 );
camera.lookAt( new THREE.Vector3( 0, 0, 0 ) );

var scene = new THREE.Scene();
var material = new THREE.LineBasicMaterial( { color: 0x0000ff } );
var geometry = new THREE.Geometry();
geometry.vertices.push(new THREE.Vector3( -10, 0, 0) );
geometry.vertices.push(new THREE.Vector3( 0, 10, 0) );
geometry.vertices.push(new THREE.Vector3( 10, 0, 0) );

var line = new THREE.Line( geometry, material );
scene.add( line );
renderer.render( scene, camera );
</script>
</html>