To use the Spine Runtimes in a way that distributes them to other people, you need a Spine license.

The Spine Runtimes license text is here:
Spine Runtimes License Agreement

When you have a Spine license, Section 2 of the Spine Editor license applies:
Spine Editor License Agreement: s2

Sorwidir wrote

I want to add, if the .js and .css is the only files needed, I assume a license wont be needed to display the sprites on the web?

You can of course display your images on a web site, but to use the Spine Runtimes to do so, you need a Spine license. The reason is that the Spine Runtimes require an enormous amount of effort to create and maintain and we do that specifically for Spine licensees.

You can't use the 4.0 runtimes with 3.7 data. See here:
Versioning - Spine User Guide: Synchronizing versions

We provide the source code for the Spine Runtimes and ask that people don't use them unless they have a Spine license. Using them anyway is illegal as it violates our licensing terms, linked above.

If you wish to make use of our software (whether the editor or the runtimes) as well as make use of the customer support we provide, we kindly ask that you purchase a Spine license. Spine Essential, while not cheap, is the lower cost option. Your Spine license is significant in that it gives you ALL future updates we provide, and we do a lot of updates.

Sorwidir wrote

Moving on, before I buy can you guarantee me that I will be able to load the sprite or art right into web without having to do any changes, just like how Skeleton Viewer can? Or am I gonna have to load all my characters (over 100), manually unpack the textures, then assign them one by one?

If you have the Spine project file(s) you can export using 4.0 and use the latest Spine player. It's been improved a lot since 3.7.

If you have only the exported JSON or binary and atlas files, you'll need to import the data using 3.7.94 (or whatever version the data is from) and save a project file. Then, since you have a project file, you can do as above to export 4.0 data. You can use the CLI to automate much of that process if you have many files:
Command line interface - Spine User Guide

You can also unpack the atlas so you have individual image files, if you want to use the editor to view the skeletons. If you don't need that, you can use the old atlases with the 4.0 data.

If you don't want to export with 4.0, you can try to use the 3.7 runtimes/player. Look at the 3.7 readme for what it supports:
https://github.com/EsotericSoftware/spine-runtimes/tree/3.7/spine-ts
That says:

spine-ts does not yet support loading the binary format.

If your data files are binary (often .skel) that means you can't use them with 3.7 spine-ts, which the player is based on. You'll have to go through the process described to import, save a project file, and export with 4.0.

Sorwidir wrote

So the reason the Skeleton Viewer can see my characters is because it has support to view binary files (.skel)?

Yep!

Sorwidir wrote

And once I buy the license, you recommend me to import the characters then export with 4.0?

Yes, then you can 1) use the 4.0 web player, and 2) edit your skeletons in the Spine editor, if you unpack the atlases.

Sorwidir wrote

But if I remember correctly, Misaki had to export the atlas, wont I have to manually assign textures to every part of the character?

There is no manually assigning textures in Spine. Spine uses naming conventions to find image files:
Images - Spine User Guide: Image file lookup

You need individual image files for the Spine editor to show your images. If all you have is an atlas, you need to unpack it:
Texture packing - Spine User Guide: Texture Unpacker

Sorwidir wrote

So with over 100 characters, it will take a long time? Or is this process also possible to "automate" with this CLI you speak of?

You can write scripts to do the steps. For example:

spine -u 3.7.94 -i old-data.skel -o new-project.spine 

---

import
spine -u 4.0.xx -i new-project.spine -o path/to/output/folder 

---

export binary

This runs 3.7.94, imports a binary file and saves a project file. Then it runs the latest 4.0 and exports the project file, giving you a binary file for each skeleton in the project.

You can also unpack an atlas:

spine -i path/to/atlas/images -o path/to/output/folder 

---

unpack path/to/your-atlas.atlas

Sorwidir wrote

I will buy a license nevertheless! Got the money now, so kinda excited.

Great! Welcome to Spine!

Those are only warnings. Your data was exported without nonessential data, so Spine has to make some guesses when importing it:
Import - Spine User Guide: Nonessential data

I don't see any attachments on your skeleton. Maybe you need to show a skin?

Ah, if your skeletons are using Spine Professional features (like meshes) then they can't be exported with Essential. I didn't know what features your skeletons use, but sorry I didn't consider that possibility earlier.

Though I understand budget is a concern, you can upgrade to Professional any time for the difference in cost from your Spine license page. If you'd prefer a refund that is no problem, in that case please email us: contact@esotericsoftware.com

Sorry!

The Skeleton Viewer source code is here:
https://github.com/EsotericSoftware/spine-runtimes/tree/4.0/spine-libgdx/spine-skeletonviewer
It could be modified to render with a green background, or to write a PNG, but would require programming with Java and libgdx.

Yep, it's normal Java development, though using the libgdx library (which Spine is built with).
https://libgdx.com/

Yep, the refund is for the full amount. You get the full amount back, but we have to pay a fee. For an Essential refund we are charged ~$3.

We lose on a refund. It's OK though, if someone finds out they really don't want Spine, they shouldn't have to pay. We should make a better product so no one wants a refund. In your case, I should have thought to mention that Essential can't export meshes.