Just wondering...
Which library sketchfab use to display the 3D models? For example: Link
HTML5 canvas of course. Looking in the code I can not understand if they use an external library or if they have written the code themselves.
Actually they seem to use a custom packed library for 3D embedding (on the example link posted)
https://d2f25wgezub9nf.cloudfront.net/builds/embed-ad258a51926008151f3c.js
the editor is this custom packed file
https://d2f25wgezub9nf.cloudfront.net/builds/editor-67a44f4ae306e7bd78a3.js and the viewer is this custom packed file
https://d2f25wgezub9nf.cloudfront.net/builds/fallbackGeneratorViewer-387bdce98e18a6654652.js
page source code:
<script type="text/javascript">
window['webpackManifest'] = JSON.parse('{
"0":"0-1c967b67206c448cd138.js",
"1":"1-0d297699ce16c718e4aa.js",
"2":"2-05aa8ecb5962ae8dbd5d.js",
"3":"3-429e7e43701ea9b55d25.js",
"5":"5-0204c61b5399fe84f239.js",
"6":"6-763c822a0a2d726a0036.js",
"11":"11-1104121b7a7583fcc8a5.js",
"12":"viewer-e459b42392a044580f6f.js",
"13":"website-513858c9e68720acb02c.js","14":"home-18d7ffed32631755f16e.js",
"15":"editor-67a44f4ae306e7bd78a3.js",
"16":"embed-ad258a51926008151f3c.js",
"17":"playlists-fff084b1baac932dd5f2.js",
"18":"fallbackGeneratorViewer-387bdce98e18a6654652.js"}');
<script src="https://d2f25wgezub9nf.cloudfront.net/builds/commons-7433f95ed304266e4ba3.js"></script>
<script src="https://d2f25wgezub9nf.cloudfront.net/builds/embed-ad258a51926008151f3c.js"></script>
</script>
Moreover sketchfab uses its own 3D viewer (https://d2f25wgezub9nf.cloudfront.net/api/sketchfab-viewer-1.0.0.js) as seen in https://sketchfab.com/developers/viewer
According to wikipedia sketchfab uses its own 3D viewer library based on the OSG.JS JavaScript library
These should answer your question