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#110 in Parser implementations

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MIT license

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docs.rs crates.io

html5gum is a WHATWG-compliant HTML tokenizer.

use std::fmt::Write;
use html5gum::{Tokenizer, Token};

let html = "<title   >hello world</title>";
let mut new_html = String::new();

for token in Tokenizer::new(html).infallible() {
    match token {
        Token::StartTag(tag) => {
            write!(new_html, "<{}>", String::from_utf8_lossy(&tag.name)).unwrap();
        Token::String(hello_world) => {
            write!(new_html, "{}", String::from_utf8_lossy(&hello_world)).unwrap();
        Token::EndTag(tag) => {
            write!(new_html, "</{}>", String::from_utf8_lossy(&tag.name)).unwrap();
        _ => panic!("unexpected input"),

assert_eq!(new_html, "<title>hello world</title>");

What a tokenizer does and what it does not do

html5gum fully implements 13.2.5 of the WHATWG HTML spec, i.e. is able to tokenize HTML documents and passes html5lib's tokenizer test suite. Since it is just a tokenizer, this means:

  • html5gum does not implement charset detection. This implementation takes and returns bytes, but assumes UTF-8. It recovers gracefully from invalid UTF-8.
  • html5gum does not correct mis-nested tags.
  • html5gum does not recognize implicitly self-closing elements like <img>, as a tokenizer it will simply emit a start token. It does however emit a self-closing tag for <img .. />.
  • html5gum doesn't implement the DOM, and unfortunately in the HTML spec, constructing the DOM ("tree construction") influences how tokenization is done. For an example of which problems this causes see this example code.
  • html5gum does not generally qualify as a browser-grade HTML parser as per the WHATWG spec. This can change in the future, see issue 21.

With those caveats in mind, html5gum can pretty much parse tokenize anything that browsers can.

The Emitter trait

A distinguishing feature of html5gum is that you can bring your own token datastructure and hook into token creation by implementing the Emitter trait. This allows you to:

  • Rewrite all per-HTML-tag allocations to use a custom allocator or datastructure.

  • Efficiently filter out uninteresting categories data without ever allocating for it. For example if any plaintext between tokens is not of interest to you, you can implement the respective trait methods as noop and therefore avoid any overhead creating plaintext tokens.

See the custom_emitter example for how this looks like in practice.

Other features

  • No unsafe Rust
  • Only dependency is jetscii, and can be disabled via crate features (see Cargo.toml)

Alternative HTML parsers

html5gum was created out of a need to parse HTML tag soup efficiently. Previous options were to:

  • use quick-xml or xmlparser with some hacks to make either one not choke on bad HTML. For some (rather large) set of HTML input this works well (particularly quick-xml can be configured to be very lenient about parsing errors) and parsing speed is stellar. But neither can parse all HTML.

    For my own usecase html5gum is about 2x slower than quick-xml.

  • use html5ever's own tokenizer to avoid as much tree-building overhead as possible. This was functional but had poor performance for my own usecase (10-15x slower than quick-xml).

  • use lol-html, which would probably perform at least as well as html5gum, but comes with a closure-based API that I didn't manage to get working for my usecase.


Why is this library called html5gum?

  • G.U.M: Giant Unreadable Match-statement

  • <insert "how it feels to chew 5 gum parse HTML" meme here>


Licensed under the MIT license, see ./LICENSE.