I'm fairly new to Blender but have already realised what an awesome combination Blender and Sketchfab make. There are a lot of features in Blender which Sketchfab doesn't support. I thought it'd be a good idea to start a topic where people share how they managed to get certain features of Blender working in Sketchfab. I'll start off with a few I've discovered in the last few weeks while learning Blender and hope others will share their tips. This is not meant to be about how to do things in Blender just how to get them to Sketchfab. I know some of these may be obvious to experienced users but not to newbies so hopefully this will help people like myself. Not sure this thread should be in support so please move if needed.
Sketchfab does not support Blender's cloth simulation modifier so you have to turn the animation into shape keys. Once you're happy with your cloth simulation, select the object then export as a Lightwave Point Cache (.mdd). To do this you need to enable Lightwave Objects and NewTek MDD format in import/export add-ons in user prefs. Now remove the cloth simulation from your object and, with it still selected, import the .mdd file you created. Animation is now in shape keys which work in Sketchfab. Example: https://skfb.ly/VNr6
Sketchfab doesn't support Blender physics simulations (as far as I know) so simply turn your simulation into keyframes which work fine in Sketchfab. Example: https://skfb.ly/VIGB
Figured this out after reading various entries on the forum. At the moment Sketchfab doesn't support camera movement easily. One solution is to move the objects in Blender to simulate camera movement. I create an empty in my scene and make it the parent of all objects in the scene. Once I'm happy with my animation, I turn on keyframe recording then go through the timeline moving, rotating and scaling the parent empty to simulate camera movement. I only got this working this morning so the example is very jerky but you get the idea. Example: https://skfb.ly/VWNI
Textures with transparency
This is probably not the right way to do it but it's the only way I could get it to work. If there's a better way, please correct me. I modelled a lego man and the textures for the mouth and eyes were pngs with transparency. I opened these in photoshop and changed the transparent areas to the colour I wanted the head to be, making note of the hex colour code used. In sketchfab I made the head material the same colour code then made sure the head, mouth and eyes material all had the same metal and roughness settings. I'm sure there's a better way but this is quick and works for me. Example: https://skfb.ly/VUKC
I ran into an issue where I'd duplicated and mirrored an object in Blender but the materials were strange in Sketchfab. Shaderbytes on the forums explained what I did wrong. In mirroring I'd set the X scale to -1. When Sketchfab reset the scale the normals were reversed. Solution is to always apply scale when mirroring objects then reverse the normals. I found it's always good practice to apply scales in Blender prior to upload. As shaderbytes pointed out, always look at your models with viewport shading set to solid. This will show up any normals issues.
An obvious one to experienced users but maybe not to newbies like me. I think Sketchfab applies smoothing and this can sometimes make flat faces look terrible in Sketchfab especially if your mesh is triangles. I get round this by adding very small bevels to the sharp edges round the face. You could also apply the Edge Split modifier which seems to do the trick too.
That's my top tips so far. I really hope others will share theirs. Anyone managed to fade an object in an animation in Sketchfab?