[Sync to prerelease] Add an axe baseline for known third-party and upstream findings

.claude/rules/axe-baseline.md

@@ -0,0 +1,34 @@
+---
+paths:
+  - "_axe-baseline.json"
+---
+
+# Axe baseline
+
+`quarto call axe` reads `_axe-baseline.json` with a strict schema. Do not add keys that the schema does not allow, for example comments.
+
+## Notes
+
+Write each `note` in this format:
+
+```
+<type>: <issue-ref>. <Description>.
+```
+
+- `<type>` is `upstream`, `third-party`, `false-positive`, or `intentional`.
+- For `third-party`, put the library name after the type: `third-party: Observable Plot, observablehq/plot#2018.`
+- Write every issue reference as `org/repo#number`, also inside the description. Do not write `quarto-cli#123` or `#123`.
+- If no issue exists, write `no upstream issue` in place of the reference.
+- Do not put the description in parentheses.
+- Do not use Markdown links or URLs. An extension makes links from the `org/repo#number` text.
+- Give the cause and the condition to remove the entry. Do not add commentary, for example "open since 2023" or "by design, not oversight".
+- If the entry is limited to some pages, give the reason: the signature that also matches content that quarto-web can fix.
+
+## Pages
+
+- Use `"pages": []` (all pages) only when the selector matches nothing but the third-party or Quarto markup.
+- If the selector is generic, for example `pre`, `th`, `a`, or `.collapsed`, list the pages.
+
+## Order
+
+The scanner does not use the order of the entries. For human readers, keep entries grouped by owner, in the order: upstream Quarto by issue, then reveal.js, then each third-party library, then false positives, then intentional examples.

README.md

@@ -187,6 +187,22 @@ quarto preview --profile prerelease,prerelease-docs
 
 Our GitHub Action workflows are documented in [`.github/workflows/README.md`](.github/workflows/README.md)
 
+## Accessibility baseline
+
+`_axe-baseline.json` lists the accessibility findings that `quarto call axe` reports as known, not new. Each entry is a finding that quarto-web cannot fix in its own content, a false positive, or an intentional example.
+
+Each entry has a `note` in this format:
+
+```
+<type>: <issue-ref>. <Description>.
+```
+
+- `<type>` is `upstream`, `third-party`, `false-positive`, or `intentional`. For `third-party`, add the library name: `third-party: leaflet, <issue-ref>.`
+- `<issue-ref>` is a full `org/repo#number` reference, for example `quarto-dev/quarto-cli#14378`. If no issue exists, write `no upstream issue`.
+- The description gives the cause and the condition to remove the entry. If the entry is limited to some pages, it also gives the reason.
+
+Do not use Markdown links in a note. Do not add commentary, for example the age of an issue.
+
 ## Style Guide
 
 You can find some style guidance in [_style-guide.md](_style-guide.md).