The WEBGL_compressed_texture_s3tc extension is part of the WebGL API and exposes four S3TC compressed texture formats.

MDN Reference

interface WEBGL_compressed_texture_s3tc {
    COMPRESSED_RGB_S3TC_DXT1_EXT: 33776;
    COMPRESSED_RGBA_S3TC_DXT1_EXT: 33777;
    COMPRESSED_RGBA_S3TC_DXT3_EXT: 33778;
    COMPRESSED_RGBA_S3TC_DXT5_EXT: 33779;
}

Properties

COMPRESSED_RGB_S3TC_DXT1_EXT: 33776
COMPRESSED_RGBA_S3TC_DXT1_EXT: 33777
COMPRESSED_RGBA_S3TC_DXT3_EXT: 33778
COMPRESSED_RGBA_S3TC_DXT5_EXT: 33779