`html_rewriter_treats_esi_include_as_void_tag` compatibility_flag not taking effect in workerd

[timkelty]

What versions & operating system are you using?

  System:
    OS: macOS 26.1
    CPU: (10) arm64 Apple M1 Max
    Memory: 245.25 MB / 32.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 22.17.1 - /Users/timkelty/.nvm/versions/node/v22.17.1/bin/node
    npm: 10.9.2 - /Users/timkelty/.nvm/versions/node/v22.17.1/bin/npm
    pnpm: 8.15.4 - /opt/homebrew/bin/pnpm
  npmPackages:
    @cloudflare/vitest-pool-workers: ^0.10.6 => 0.10.7
    wrangler: ^4.47.0 => 4.48.0

Please provide a link to a minimal reproduction

https://github.com/timkelty/cloudflare-esi-compat-flag-bug

Describe the Bug

Compatibility flags from wrangler.jsonc not applied in vitest-pool-workers test environment

Description

The html_rewriter_treats_esi_include_as_void_tag compatibility flag specified in wrangler.jsonc is not being applied when running tests with @cloudflare/vitest-pool-workers. This causes HTMLRewriter to throw a TypeError: Parser error: Unsupported pseudo-class or pseudo-element in selector when trying to use esi:include as an element selector.

See reprocase: https://github.com/timkelty/cloudflare-esi-compat-flag-bug

[petebacondarwin]

I can't get this flag to work in a normal Worker either. Can you?
For example I copied the test code into the src/index.ts file and then ran npx wrangler dev.
It fails with the same error.

[petebacondarwin]

I am confident that flags are being passed from the wrangler.json through vitest-pool-workers and miniflare to the workerd instance. So I propose that this is actually an issue with workerd. So I am going to transfer this over to workerd.

[timkelty]

Makes sense, thanks @petebacondarwin!

[timkelty]

@petebacondarwin You're right – it appears to not work at all, even when deployed I'm getting the same error.

https://github.com/timkelty/esi-html-rewriter