ReadonlyCOMPRESSED_RGBA_S3TC_DXT1_EXT
COMPRESSED_RGBA_S3TC_DXT1_EXT: 33777
ReadonlyCOMPRESSED_RGBA_S3TC_DXT3_EXT
COMPRESSED_RGBA_S3TC_DXT3_EXT: 33778
ReadonlyCOMPRESSED_RGBA_S3TC_DXT5_EXT
COMPRESSED_RGBA_S3TC_DXT5_EXT: 33779
ReadonlyCOMPRESSED_RGB_S3TC_DXT1_EXT
COMPRESSED_RGB_S3TC_DXT1_EXT: 33776
The WEBGL_compressed_texture_s3tc extension is part of the WebGL API and exposes four S3TC compressed texture formats.
MDN Reference