For Docutils, PIL is a recommendation, no requirement.
It is, however, required for the extraction of the image size
from an image file and a warning is written if it it missing.
This makes the output differ from the "expected" sample on
a functional test where the "scale" option of the "image"
directive is used.
The "length_units_html5" functional test is intended to provide
an example of the length unit support for visual inspection with
a HTML browser. There are additional unit tests for the
"scale" option with and without PIL, so we can safely skip
the functional test if PIL is missing on a system.