  Electromagnetic plane waves, solutions to Maxwell's equations, are said to be
`transverse' in vacuum. Namely, the waves' oscillatory electric and magnetic
fields are confined within a plane transverse to the waves' propagation
direction. Under tight-focusing conditions however, the field can exhibit
longitudinal electric or magnetic components, transverse spin angular momentum,
or non-trivial topologies such as M\"obius strips. Here, we show that when a
suitably spatially structured beam is tightly focused, a 3-dimensional
polarization topology in the form of a ribbon with two full twists appears in
the focal volume. We study experimentally the stability and dynamics of the
observed polarization ribbon by exploring its topological structure for various
radii upon focusing and for different propagation planes.
