Request context: [[ https://upload.wikimedia.org/wikipedia/commons/c/c9/Ballot-sprite.svg | Loading SVG ]] as background image from [[ https://commons.wikimedia.org/w/index.php?title=MediaWiki%3AGadget-EnhancedPOTY.css&type=revision&diff=191359963&oldid=114497048 | this CSS file ]].
GET /wikipedia/commons/c/c9/Ballot-sprite.svg HTTP/1.1
Host: upload.wikimedia.org
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:45.0) Gecko/20100101 Firefox/45.0
**Accept: image/png,image/*;q=0.8,*/*;q=0.5**
Accept-Language: de,en-US;q=0.7,en;q=0.3
Accept-Encoding: gzip, deflate, br
Referer: https://commons.wikimedia.org
Cookie: WMF-Last-Access=26-Mar-2016;<secret>
Connection: keep-alive
Pragma: no-cache
Cache-Control: no-cache
HTTP/1.1 200 OK
Date: Sat, 26 Mar 2016 22:43:24 GMT
**Content-Type: text/html**
Content-Length: 1029
x-object-meta-sha1base36: <some-stuff-i-cant-read>
Last-Modified: Sat, 26 Mar 2016 22:19:52 GMT
Etag: <more-stuff-i-cant-read>
X-Timestamp: 1459030791.20288
x-trans-id: <more-stuff-i-cant-read>
Content-Encoding: gzip
X-Varnish: <more-stuff-i-cant-read>
Via: 1.1 varnish, 1.1 varnish, 1.1 varnish
Accept-Ranges: bytes
Age: 1380
X-Cache: cp1050 miss(0), cp3049 hit(1), cp3039 frontend hit(13)
strict-transport-security: max-age=31536000
x-analytics: WMF-Last-Access=26-Mar-2016;https=1
x-client-ip: 217.186.201.91
access-control-allow-origin: *
access-control-expose-headers: Age, Date, Content-Length, Content-Range, X-Content-Duration, X-Cache, X-Varnish
Timing-Allow-Origin: *
X-Firefox-Spdy: 3.1
Expected: content-type: **image/svg+xml**
Therefore, I can't use uploaded SVGs as background images for POTY. Firefox simply doesn't display it. It says an error occurred somewhere on some background property but the developer tools are currently broken and do not display the correct location (always show something in OOjs UI).