WebGL自由研究。Suzanne.gltf専用ローダ(viewer)54行。

gltfLoader.html

<canvas width="256" height="256"></canvas><hr/>
<script>"use strict";
let cx=document.querySelector("canvas").getContext("webgl");
let vsrc=`attribute vec4 p,n,uv;uniform mat4 m;varying vec3 vn;
void main(){gl_Position=p*m;gl_Position.xy/=1.+gl_Position.z*.5;vn=(n*m).xyz;}`;
let fsrc=`precision highp float;varying vec3 vn;
void main(){gl_FragColor=vec4(vec3(1)*dot(vec3(-.5),vn),1);}`;
let pg=cx.createProgram();{
	let vs=cx.createShader(cx.VERTEX_SHADER);cx.attachShader(pg,vs);
	let fs=cx.createShader(cx.FRAGMENT_SHADER);cx.attachShader(pg,fs);
	cx.shaderSource(vs,vsrc);cx.compileShader(vs);
	cx.shaderSource(fs,fsrc);cx.compileShader(fs);cx.linkProgram(pg);
};	cx.enable(cx.DEPTH_TEST);
let bf=cx.createBuffer(), ef=cx.createBuffer();
let gltf=[],vNum=0, AB=cx.ARRAY_BUFFER,EB=cx.ELEMENT_ARRAY_BUFFER;
let ldBf=(b)=>{
	cx.bindBuffer(AB,bf);
	cx.bufferData(AB,b,cx.STATIC_DRAW);
	cx.bindBuffer(AB,null);
	cx.bindBuffer(EB,ef);
	cx.bufferData(EB,b.slice(62912,62912+5808),cx.STATIC_DRAW);
	cx.bindBuffer(EB,null);
}
let ldJson=(r)=>{
	fetch(r.buffers[0].uri).then(r=>r.arrayBuffer()).then(ldBf);gltf=r;
}
let ldAcc=()=>{
	for(let i=0;i<gltf.accessors.length;i++){
		let bufferView=gltf.accessors[i].bufferView;
		let type=gltf.accessors[i].type;
		let byteOffset=gltf.bufferViews[bufferView].byteOffset;
		let size={"SCALAR":1,"VEC2":2,"VEC3":3,"VEC4":4}[type];
		if(size!=1){
			cx.bindBuffer(AB,bf);
			cx.enableVertexAttribArray(bufferView);
			cx.vertexAttribPointer(bufferView,size,cx.FLOAT,false,0,byteOffset);
			cx.bindBuffer(AB,null);
		}
	};	vNum=gltf.accessors[3].count;
}
let mp=cx.getUniformLocation(pg,"m");
let m=new Float32Array([1,0,0,0,0,1,0,0,0,0,1,0,0,0,0,2]);
let th, cos=Math.cos;
let draw=(now)=>{
	th=now/1500, m[0]=m[10]=cos(th), m[2]=cos(th+Math.PI/2), m[8]=-m[2];
	cx.useProgram(pg);	cx.uniformMatrix4fv(mp,false,m);
	cx.clearColor(0,0,.1,1);cx.clear(0x4000);cx.bindBuffer(EB,ef);{
		cx.drawElements(cx.TRIANGLES,vNum,cx.UNSIGNED_SHORT,0);
	};cx.bindBuffer(EB,null);
	requestAnimationFrame(draw);
}
let ftJson=f=>fetch(f).then(r=>r.json()).then(ldJson);
ftJson("suzanne.gltf").then(ldAcc).then(draw);
</script>

多分arrayBufferを拾う練習がテーマだったので、bufferのoffset等の設定は手動です。
gltfファイルから読むか、JSON型変数"gltf"から拾ってください。
CORS云々はVS CodeのHTML viewerとかでやると楽かもです。