Mod 'OptiFabric' (optifabric) requires any version between 0.8.0 (inclusive) and 0.12 (exclusive) of mod 'Fabric Loader' (fabricloader), but only the wrong version is present: 0.12.5!
Mod 'Fabric Loader' (fabricloader) 0.12.5 is being loaded from

I am getting the same error

Same here

The easiest solution is to modify the loading file for OptiFabric so that's it's compatible with all versions.

How would I acomplish that?

Same here. Please fix :)

How would I acomplish that?

You'd open the jar file, open the fabric.mod.json file and make these changes:

-34   "fabricloader": ">=0.8.0 <0.12",
+34   "fabricloader": ">=0.8.0 <0.13",

After these changes the mod will load. but there's still no chance that it'll detect modern versions of OptiFine.

This fabricloader limit redefining helped. I may not be able to use optifine, but it did help fix compatibility with another mod. Appleskin needed Fabric 0.12 but this mod didn't support it yet (at least, it said it didn't), but @cameronPeckDev 's fix helped.
Nevermind; it may have started but probably because of Fabric >0.11.7, it doesn't start up at all. It just doesn't give an error either. This means optifine doesn't work (yet) either

same as #394