Minecraft wiki entry for Opacity, associated text: In the Nether, the spawn point is always on top of an opaque block. This can be seen if the player used a respawn anchor to set their spawn in the Nether.
If an opaque block is given transparent pixels in an edited texture, then instead of showing the block behind it through the transparent part, it simply fills in the area with white.[verify] However, currently X-ray texture packs could be created using the blocks custom models feature.[1]