Parsoid/Parser Unification

Longer Term Goal: Parsoid is the default wikitext engine for MediaWiki and the legacy parser is removed from the codebase

Intermediate Goal: Parsoid replaces the core parser for all wikitext use cases on the Wikimedia cluster.

• Parser tests: This is how Parsoid has been developed since its inception. We ensure that Parsoid continues to pass parser tests, and where divergence is known, it is recorded after careful review. We have also vastly expanded parser test coverage over the years, and all patches against Parsoid need to pass tests.
• Round-tripping / Integration tests: In this mode, before every production deployment, we convert wikitext to HTML and HTML back to wikitext on about 180K pages from about 50 production wikis. While this testing mode is primarily to ensure our HTML -> wikitext conversion is not broken (which would impact our editing client tools), this also implicitly serves to flag any breakages in our HTML output. But, these aren't the most reliable tests for verifying that our HTML output is not broken.
• Visual diff tests: Here, we take renderings of legacy parser HTML and Parsoid HTML and compare the rendering screenshots and generate a numeric diff score. We have run this in an automated way on 25k+ pages from about 20 production wikis. This has been a really reliable way to identify various breakages and bugs in Parsoid output. As we get closer to rollout, we intend to expand our testing to a wider range of wikis.
• Parsoid reading and editing clients: Parsoid's output has been used over the years by VisualEditor, Android and iOS mobile apps, Kiwix, and other clients. We have fixed a number of bugs and incompatibilities in Parsoid over the years and continue to fix the various long-tail edge cases as they are discovered and reported.

As we get closer to rollout, we will identify other QA and testing methodologies as required to ensure we can roll out this change in as smooth and non-disruptive fashion as possible and will update this page as that happens.

At this stage of this project, we have split this work into a number of steps to achieve the intermediate goal.

1. ✅ Deploy changes to core that makes media structure HTML largely identical to what Parsoid emits. This has its own deployment plan. This change has been live on mediawiki.org and officewiki since September 2021 and we expect to roll this out to all wikis gradually in 2022.
2. ✅ Deploy individual user opt-in tools to use Parsoid for read views as part of the ParserMigration extension.
3. ✅ Deploy changes to Wikimedia production that lets DiscussionTools use Parsoid HTML directly. This lets us iron out bugs in a restricted use case.
4. Turn on Parsoid HTML read views on additional wikis incrementally
   • ✅ officewiki
   • ✅ Talk pages on wikitech
   • (in progress) wikivoyage
   • ...
5. Continued work to ensure Parsoid is able to generate identical metadata that the legacy parser generates (categories, backlink tables, page properties, etc). This is needed for tighter integration of Parsoid into MediaWiki core and to start replacing the legacy parser in additional wikitext use cases.
6. Start rolling out on all wikis gradually -- more specific deployment plan will be developed based on what we learn in previous stages.

To validate our road-map evolution and use data-driven decision making for deployments, we have developed a Confidence Framework for Parsoid Read Views. This framework contains the guidelines for how we prioritise features, bugfixes, and deployments.

For the most part, the switch to Parsoid generated HTML should be transparent to most users. But, below, we outline some possible impacts on readers, editors, and developers.

Parsoid models and processes wikitext differently compared to the legacy parser and this can sometimes lead to differences in rendering in some edge case scenarios. If some wikitext pattern is commonly used, we have attempted to support that in Parsoid where possible, and where not, by either fixing or providing support to fix them up. At this time, we believe all rendering differences we expect to run into will be edge cases that can likely be adjusted by fixing wikitext either on individual pages or on templates.

• Parsoid's HTML for media wikitext is different from what the legacy parser has typically generated. As part of a separate project to use semantic HTML5 output for images, the legacy parser is currently being updated to generate HTML that is pretty close to Parsoid's HTML. We expect to roll this out this year which might require some skins, gadgets, bots, and template styles to be updated.
• The Cite extension that targets Parsoid relies on CSS rules to localize numbering of references rather than generate localized HTML. This requires editors with appropriate permissions to update MediaWiki:Common.css on their wikis to add suitable CSS rules targeting this HTML.

Parsoid's internal processing model is different from the legacy parser. As a result, extensions may need to be updated. This only impacts extensions that do one or more of the following: (a) operate on wikitext (b) provide handlers for parser hooks (c) call a public method of the legacy parser.

Extensions that process wikitext will definitely need to be updated to work with Parsoid. To date, the vast majority of such extensions have been updated. Since Parsoid continues to access the legacy parser for expanding templates, processing parser functions, any parser hooks triggered during this processing will continue to operate and extensions that rely on these hooks will continue to operate. For the rest, we are exploring strategies to minimize updated needed to extensions.

We will file phabricator tasks for all impacted extensions as we proceed with this work, and will fix whatever extensions we can within our team. If you are an extension developer, we would greatly appreciate any proactive work and code review (for patches we might submit).

Kartographer has been ported to Parsoid, but the port is not deployed to all wikis yet. It can be deployed on a specific wiki with a patch similar to Gerrit change 969168. It is strongly advised to prepare a list of pages to test during the deployment (via Wikimedia Debug) and to test these before moving on with the full deployment. The following cases are interesting to test:

• Map in an infobox (exemple https://en.wikipedia.org/wiki/Z%C3%BCrich_District)
• Single map outside of an infobox (example https://en.wikipedia.org/wiki/Z%C3%BCrich_S-Bahn)
• Page with external data (example https://en.wikipedia.org/wiki/Breaksea_Sound)
• Page with maplinks (example https://en.wikipedia.org/wiki/List_of_lighthouses_in_England)
• Page with multiple mapframes (example https://en.wikipedia.org/wiki/Long_Island_Motor_Parkway)
• Wikivoyage: Page with multiple markers and multiple groups (example: https://en.wikivoyage.org/wiki/Canary_Islands)
• Wikivoyage: page with multiple maps (example https://en.wikivoyage.org/wiki/Paris)
• Wikivoyage: Page with maplinks without a mapframe (example https://en.wikivoyage.org/wiki/Adelaide_Hills)

The Content Transform Team is driving this project. Our goal is to make this switch to Parsoid as seamless as possible. So, we have tried to roll out changes over the years gradually.

We started with replacing HTML4 Tidy with HTML5 RemexHtml in the 2015 - 2018 timeframe. In 2019, in preparation to integrate Parsoid into MediaWiki core more closely, we ported Parsoid from JS to PHP. This switch went very smoothly. In the 2020 - 2022 timeframe, we started work to unify the media output generated by Parsoid and by core. This has mostly involved making changes to core, but we have occasionally adjusted Parsoid's output based on feedback and other technical considerations.

Going forward, we will provide support in the following ways:

• Linter rules for any wikitext that needs fixing.
  • The vast majority of this work was completed as part of the Tidy -> Remex migration and we don't expect to introduce a large number of new linter categories for this
• Communication via this page, via tech news updates, and via updates and posts to village pump and other wiki-specific forums.
• Opt-in mechanisms for early adopter users / wikis to test and report problems.
  • See the next section for more details!

Starting November 2023, you can opt-in to using the new Parsoid parser for reading articles on Wikipedia. See Help:Extension:ParserMigration for more information!

Other things you can do to help:

• Test your gadgets / user scripts against Parsoid HTML to identify / fix any breakages
  • Use the ParserMigration extension to allow you to easily swap between legacy and Parsoid HTML
• Parsoid read views will be rolled out first on wikis whose communities have elected to be early adopters; watch this space for more details.

• Parsoid/Parser Unification/Updates Project updates
• Parsoid/Parser Unification/Known Issues lists differences between the Parsoid parser and the legacy parser which a user is likely to notice when using the ParserMigration extension.
• Parsoid Performance Considerations outlines performance work needed for Parsoid and acceptance criteria for deploy
• Historical documents:
  • Known differences between Parsoid & core parser output is a more technical page listing specific differences in HTML output, of interest to developers and authors of CSS styles. This page is not as relevant anymore. We are now focused on more fine-grained problems found during visual diff testing.
  • The Pixel_Diff_Testing_Stats page tracked progress in measuring (and achieving) rendering parity between Parsoid and core parser output. This page is no longer relevant right now because we are no longer doing generic visual diff tests but instead focusing on wikis we are rolling out to and doing focused visual diff tests for those wikis and fixing problems we find.
  • February 2019 tech talk: The long and winding road to making Parsoid the default MediaWiki parser
  • Replacing Tidy: Project page related to replacing Tidy
  • Parsing/Notes/Two Systems Problem. This 2016 document explored different options at arriving at a single parser
  • Parsing/Notes/Moving Parsoid Into Core