Henri Sivonen
Senior Staff Software Engineer at Mozilla
Helsinki, Finland
Summary
🤩
Rockstar🎓
Top SchoolHenri Sivonen is a Senior Staff Software Engineer in Helsinki with 20 years of experience building foundational pieces of the web stack, currently owning Firefox’s HTML parser, character encoding converters, and detectors. He combines deep systems and standards expertise—having represented Mozilla on the W3C HTML Working Group—with hands-on open source contributions to projects like Gecko, ICU/ICU4X and web-platform-tests. His work spans low-level Unicode and collation plumbing to browser internals (including focus management for out-of-process iframes) and practical interoperability fixes such as correct about:blank handling. Comfortable operating above networking and below layout, he’s noted for integrating complex internationalization tooling into large C++ codebases while still authoring language-agnostic validator and test-suite components.
20 years of coding experience
12 years of employment as a software developer
Master of Science Computer Science, Master of Science Computer Science at Teknillinen korkeakoulu-Tekniska högskolan
Stackoverflow
7,966reputation
1.7mreached
43answers
15questions
html
top-1%
hyperlink
top-1%
browser
top-5%
cross-browser
top-5%
firefox
top-5%
xhtml
top-5%
Github Skills (45)
c-language10
rails-i18n10
testing10
css10
internationalization10
gecko10
data-structure10
icu4c10
java10
normalization10
javas10
ember-i18n10
collation10
web-standards10
unicode10
Programming languages (20)
Github contributions (5)
validator/validator
Mar 2006 - Jan 2017
Nu Html Checker – Helps you catch problems in your HTML/CSS/SVG
Role in this project:
Back-end Developer
Back-end DeveloperContributions:1 review, 958 commits, 3 PRs in 11 years
Contributions summary:Henri's contributions appear to focus on implementing core functionality for the Nu Html Checker, a tool for catching problems in HTML, CSS, and SVG. The commits demonstrate efforts to build out a new Java-based driver component and related changes to the parser's functionality. The user also introduced data attribute checks.
lintercheckerproblemscssjavascript
unicode-org/icu4x
Mar 2020 - Dec 2022
Solving i18n for client-side and resource-constrained environments.
Role in this project:Back-end Developer
Back-end DeveloperContributions:231 reviews, 52 commits, 114 PRs in 2 years 9 months
Contributions summary:Henri contributed extensively to the `icu4x` repository, focusing on implementing and refining core functionalities related to Unicode properties and collation. Their commits centered on implementing the Canonical_Combining_Class property and building the underlying code structures for properties, including trie value handling and data structures. The user further implemented a collator under experimental and enhanced this with validation and handling for various locales.
environmentssolvingclient-siderusttranslation
Find and Hire Top DevelopersWe’ve analyzed the programming source code of over 60 million software developers on GitHub and scored them by 50,000 skills. Sign-up on Prog,AI to search for software developers.