Cargo Features

sidoc-html5 has no features set by default.

[dependencies]
sidoc-html5 = { version = "0.1.2", features = ["extra-validation"] }
extra-validation = lazy_static

Features from optional dependencies

In crates that don't use the dep: syntax, optional dependencies automatically become Cargo features. These features may have been created by mistake, and this functionality may be removed in the future.

lazy_static extra-validation?