Markdown Feeds: Accept-header negotiation ignores quality values and only reads the first media range

[ekamran]

Description

With the Markdown Feeds experiment and its "Serve Markdown when a request prefers it via the Accept header" setting enabled, the decision only looks at the first media range of the Accept header. Quality values are not read. On a singular URL:

Accept header	What a client means	Served
text/markdown;q=0.9, text/html	prefers HTML	Markdown
text/markdown;q=0	refuses Markdown	Markdown
application/json, text/markdown	accepts Markdown, not HTML	HTML
text/html;q=0.1, text/markdown	prefers Markdown	HTML
TEXT/MARKDOWN	Markdown	HTML (case)

The check is preg_match( '~^text/(?:x-)?markdown(?:[,;]|$)~', $accept ) in Markdown_Feeds::is_markdown_requested().

Expected: Markdown is served when the header prefers it over HTML, as the setting's label says, and HTML in every other case. A tie, */* or text/* should keep HTML, so browsers (text/html,...,*/*;q=0.8) and curl (*/*) are not affected.

The setting is off by default and only singular URLs negotiate, so this touches few requests today. I have a fix with tests ready and will open a PR.

Step-by-step reproduction instructions

1. Enable the Markdown Feeds experiment under Settings > AI and switch on "Serve Markdown when a request prefers it via the Accept header".
2. Request a published post with curl -sI -H 'Accept: text/markdown;q=0.9, text/html' <post URL>. The response is text/markdown, although the header prefers HTML.
3. Request it with curl -sI -H 'Accept: application/json, text/markdown' <post URL>. The response is text/html, although Markdown is the only acceptable type of the two.
4. Request it with curl -sI -H 'Accept: text/markdown;q=0' <post URL>. The response is text/markdown, although the client refused it.

Screenshots, screen recording, code snippet

Output on a local site with the setting on (develop at 2cc9603):

Accept: text/markdown;q=0.9, text/html         -> text/markdown
Accept: text/markdown;q=0, text/html;q=0.5     -> text/markdown
Accept: application/json, text/markdown        -> text/html
Accept: text/html;q=0.1, text/markdown         -> text/html
Accept: TEXT/MARKDOWN ; charset=utf-8 ; q = 1  -> text/html
Accept: text/html, text/markdown               -> text/html
Accept: */*                                    -> text/html

Environment info

• WordPress 7.1, PHP 8.5.11, SQLite (local site), pretty permalinks
• AI plugin: develop at 2cc9603 (the Accept check is unchanged since 1.4.0)
• Theme: Twenty Twenty-Five
• No other plugins active

Please confirm that you have searched existing issues in the repo.

• Yes

Please confirm that you have tested with all plugins deactivated except the AI plugin.

• Yes

Please confirm which theme type you used for testing.

• Block
• Classic
• Hybrid (e.g. classic with theme.json)
• Not sure