@Krinkle out of interest why is this the status quo? It seems like this would effect SEO and be cached on the local browser.
Why do we show error pages with a 200 instead of 404? Is this documented somewhere?

@Jdlrobson It is the status quo because outputting unstyled errors is not up to our standards. It lacks user experience (no navigation options, other than browser Back). Any user-accessible page ought to rendered with our visual identity.

We don't show error pages with HTTP 200. If some cases do, that's a bug. Using OutputPage/Skin or not has no relation to the http status code:

• HTTP 400 https://en.wikipedia.org/wiki/!%3Cinvalid
• HTTP 404 https://en.wikipedia.org/wiki/!krinkle
• HTTP 404 https://en.wikipedia.org/w/index.php?title=!krinkle&action=raw
• HTTP 404 https://en.wikipedia.org/wiki/Special:Version/License/NotExist
• HTTP 429 (throttled action)
• HTTP 500 (sql error or uncaught exception)

thanks for clarification @Krinkle - is there any page documented how to do a 404 / 400 with correct styling. Special:UserProfile without parameters should never be linked to from anywhere and should not come up in search engines.

@Jdlrobson I'm not sure what kind of documentation you're looking for. Output a page like any other and set the desired status code (if other than 200, which is PHP's default). There's no special error class for it. It's part of how OutputPage and Skin work. Just like setting page title, setting robot policy, setting canonical url, etc. one can also set status code.

Change 188602 had a related patch set uploaded (by Florianschmidtwelzow):
SpecialUserProfile: Return Status code 404 when no user found

https://gerrit.wikimedia.org/r/188602

Patch-For-Review

Hey @Krinkle I was merely hoping for some kind of php doc link to the relevant function. I don't like to assume volunteers know how to do this sort of thing so any guidance no matter how trivial it seems can help someone get their first commit in but looks like @Florian has made this unnecessary :)

Change 188602 merged by jenkins-bot:
SpecialUserProfile: Return Status code 404 when no user found

https://gerrit.wikimedia.org/r/188602