The module structure is also documented in the output produced by cargo doc, alongside individual functions etc.
src/: The main html5ever library crate.
src/driver.rs: Provides the highest-level interfaces to the parser, i.e. "here's a string, give me a DOM"
src/tokenizer/: The first stage of HTML parsing, corresponding to WHATWG's section 12.2.4 "Tokenization"
src/tree_builder/: The second (and final) stage, corresponding to section 12.2.5 "Tree Construction"
src/serialize/: Turning trees back into strings. Corresponds to section 12.3 "Serialising HTML fragments"
src/sink/: Types that html5ever can use to represent the DOM, if you do not provide your own DOM implementation.
src/for_c/: Implementation of the C API for html5ever (as yet incomplete)
macros/: Rust syntax extensions used within html5ever. Users of the library do not need this crate.
capi/html5ever.h: C header for the C API
tests/: Integration tests. This is a single executable crate that runs html5ever on the various html5lib-tests. There are also unit tests throughout the library code. See README.md for information on running tests.
bench/: Benchmarks. Another executable crate.
examples/: Examples of using the library. Each .rs file is an executable crate.
data/: Various data used in building and benchmarking the parser.