I have a 2013 App / Add-in deployed in two places. The parent site app deploy looks good and inherits the Master Page and CSS.


But when I deploy that same app to a Subsite,


..no CSS is loaded and the console throws the error:

**SEC7113: CSS was ignored due to mime type mismatch**

Any ideas on what's happening?

  • Did you try in multiple browsers?For troubleshooting try in chrome if you haven't done yet
    – Unnie
    Mar 8, 2016 at 22:16
  • same issue in Chrome, similar error "Resource interpreted as Stylesheet but transferred with MIME type text/html:" Mar 8, 2016 at 23:24
  • maybe use fiddler or chrome's network tracing feature to check the http request/response for the css file?
    – Paul Lucas
    Mar 11, 2016 at 6:14

2 Answers 2


What's the CSS URL? If it's relative, the subsite might be trying to hit an invalid URL, causing an error and thus the browser is unable to interpret the response into a valid CSS file.

Check the http traffic with the Network tab or a tool like Fiddler.


From this thread at Technet Forums:

The URL of the server's domain contained an underscore ex : (http://cont_oso/) by changing it to a hypen ex : (http://cont-oso/) the problem was solved ! (also needed to change the name in the bindings)

If your subsite has any non-alphanumeric chars in URL, it might be useful to check.

  • Also, I wonder if double slash in question was a typo? > subsite//Pages Mar 15, 2016 at 16:13
  • the double slash was a typo ha. But there are no underscores or alphanumeric characters in the URL Mar 15, 2016 at 21:07
  • Ok, guess I'll leave this answer for future googlers. Did you have a chance to setup Fiddler and see the MIME code in actual response for css file? Mar 15, 2016 at 21:20
  • App.css in Fiddler is getting a 304 Result. But I'm unsure where to look for MIME within a response Mar 15, 2016 at 21:27
  • Like i.imgur.com/vrplxvM.png. Does it give 304 even when app is deployed to root? Also, could you clear browser cache and test both cases? Mar 15, 2016 at 21:51

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