Feature tags can be used in variant lists to express the quality
degradation associated with the presence or absence of certain
features. One example is
{"index.html.plain" 0.7 },
{"index.html" 1.0 {features tables frames}}
Here, the "{features tables frames}" part expresses that index.html
uses the features tagged as tables and frames. If these features are
absent, the overall quality of index.html degrades to 0. Another example is
{"home.graphics" 1.0 {features !textonly}},
{"home.textonly" 0.7 }
where the "{features !textonly}" part expresses that home.graphics
requires the absence of the textonly feature. If the feature is
present, the overall quality of home.graphics degrades to 0.
The absence of a feature need not always degrade the overall quality
to 0. In the example
{"x.html.1" 1.0 {features fonts;-0.7}}
the absence of the fonts feature degrades the quality with a factor
of 0.7. Finally, in the example
{"y.html" 1.0 {features [blebber wolx] }}