Informative

Pink and blue clouds in the Orion Nebula

The alternative conveys the observation's useful content.

Decorative

The nearby heading already supplies all meaning, so alt is empty.

Functional

Open the full Horsehead Nebula observation

The image is the link's only content, so alt names the destination.

Complex observation

Lunar terrain map; values follow in the table
Exposure measurements along the lunar terminator.
Equivalent exposure data
RegionSeconds
North rim0.008
South basin0.013