`contributes.format.html.css` from metadata extension ignored

I have:

• searched the issue tracker for similar issues
• installed the latest version of Quarto CLI
• formatted my issue following the Bug Reports guide

Bug description

An extension that uses contributes.format.html.css in its _extension.yml never has its CSS included in the rendered output when the consuming project renders to plain format: html. The CSS file is silently ignored, even when the consuming project has no css field of its own.

By contrast, contributes.metadata (e.g., project.pre-render scripts) from the same extension works as expected.

Steps to reproduce

https://github.com/posit-dev/quarto-openapi/tree/main/example

The extension at _extensions/quarto-openapi/_extension.yml declares:

  contributes:
    metadata:
      project:
        pre-render:
          - _extensions/quarto-openapi/openapi-to-markdown.ts
    format:
      html:
        css:
          - quarto-openapi-styles.css

The project's _quarto.yml is:

  project:
    type: website

  website:
    title: "Pet Store API Docs"

  openapi:
    spec: "openapi.json"
    output: "api/index.qmd"

quarto render and inspect the generated HTML.

Actual behavior

The pre-render script from contributes.metadata runs correctly but, the CSS from contributes.format.html.css is not included in the rendered HTML.

Expected behavior

The rendered HTML should include quarto-openapi-styles.css.

Your environment

• IDE: Terminal (zsh)
• OS: macOS Darwin 25.4.0

Quarto check output

❯ quarto check
Quarto 1.9.36
[✓] Checking environment information...
      Quarto cache location: /Users/edavidaja/Library/Caches/quarto
[✓] Checking versions of quarto binary dependencies...
      Pandoc version 3.8.3: OK
      Dart Sass version 1.87.0: OK
      Deno version 2.4.5: OK
      Typst version 0.14.2: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
      Version: 1.9.36
      Path: /Users/edavidaja/.local/share/qvm/versions/v1.9.36/bin

[✓] Checking tools....................OK
      TinyTeX: (external install)
      Chromium: (not installed)
      Chrome Headless Shell: (not installed)
      VeraPDF: (not installed)

[✓] Checking LaTeX....................OK
      Using: TinyTex
      Path: /Users/edavidaja/Library/TinyTeX/bin/universal-darwin
      Version: 2024

[✓] Checking Chrome Headless....................OK
      Using: Chrome found on system
      Path: /Applications/Google Chrome.app/Contents/MacOS/Google Chrome
      Source: MacOS known location

[✓] Checking basic markdown render....OK

[✓] Checking R installation...........OK
      Version: 4.5.1
      Path: /Library/Frameworks/R.framework/Versions/4.5-arm64/Resources
      LibPaths:
        - /Users/edavidaja/Library/R/arm64/4.5/library
        - /Library/Frameworks/R.framework/Versions/4.5-arm64/Resources/library
      knitr: 1.50
      rmarkdown: 2.30

[✓] Checking Knitr engine render......OK

[✓] Checking Python 3 installation....OK
      Version: 3.14.3
      Path: /opt/homebrew/opt/python@3.14/bin/python3.14
      Jupyter: (None)

      Jupyter is not available in this Python installation.
      Install with python3 -m pip install jupyter

      There is an unactivated Python environment in .venv. Did you forget to activate it?

[✓] Checking Julia installation...

Start with the reproduction in posit-dev/quarto-openapi/example, especially _extensions/quarto-openapi/_extension.yml and the consuming _quarto.yml. Run quarto render and inspect the generated HTML to trace why contributes.metadata is applied while contributes.format.html.css is not. Done means quarto-openapi-styles.css appears in the rendered HTML for plain format: html.