A very complex dielectric mirror can reflect up to 99.999% of the light incident upon it, for a narrow range of wavelengths and angles.
Almost any dielectric material can act as a perfect mirror through total internal reflection.
The effect happens when light goes from a medium with a higher index of refraction to one with a lower value (like air).
[2][3] These unusual mirrors are very efficient reflectors over a broad range of angles and wavelengths, and are insensitive to polarization.
A version of the perfect mirror that was developed at MIT for military use is used by OmniGuide in laser surgery.