This resourcepack is custom-made for shaders, so the textures might look weird if you aren’t using any shaders or shaders that doesn’t support all of the features or normal/specular mapping.
The pack focuses on large scale texture tiling / randomization using various optifine-format features instead of having high texture res or detailed custom block models. And they’re mostly color-matched to default textures to ensure existing builds and maps won’t look too different when viewed with the pack. Grass and leaves texture are also compatible with biome coloring so they should work with datapack/modded biome tints.
