With any of the following Accept headers

• text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8 (Firefox default)
• text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8 (Chromium default)
• text/html, */*;q=0.001 (reduced example)

Special:PageData redirects to the raw page instead of the HTML wiki page. The only quality values that will produce a redirect to the wiki page are 0.000 through 0.0009. I don’t think this is an actual violation of RFC 7231, since the negotiation algorithm doesn’t seem to be specified in detail, but I’m sure it’s not desirable that opening a Special:PageData URL in a standard browser doesn’t result in a nice web page.

Here are two example URLs to try this out:

• https://commons.wikimedia.org/wiki/Special:PageData/main/Data:Ncei.noaa.gov/weather/New_York_City.tab
• https://commons.wikimedia.org/wiki/Special:PageData/main/Data:Bundestagswahl2017/wahlkreis46.map