When a style has no rules, the "else" symbolizers apply. When a style has rules and none of them apply to the given feature, the "else" symbolizers apply. Note that this is different than default symbolizer properties that might be merged into all symbolizers (as in OL2) - I don't think we should support that.
Now vector layers can have a style. ol.Style instances have an
apply method to get the symbolizer literals for a feature. If the
layer does not have a style defined, there is also a static
applyDefaultStyle function on ol.Style to get the default
symbolizer literals for a feature. The vector layer also got a
groupFeaturesBySymbolizerLiteral method, which returns an array
with features grouped by symbolizer, as needed by the canvas
renderer.
