How do I investigate a "style sheet could not be loaded" message in Firefox?
How do I investigate a "style sheet could not be loaded" message in Firefox? This message appears as a red bar below the page contents and above the developer tools. How do I find out what file the browser is referring to? I'm running version 46.0 on Linux Mint 17.3.
Update
If I look at the console of the developer's tools in Firefox, it shows all the css files and says "HTTP/1.1 200 OK" against each file.
Another Update
This error bar comes and goes, it is not consistent for a particular page.
This may be a very specific edge case, but I had this exact same error with no indication as to which style sheet Firefox was complaining about. It turns out that it was the Ad Blocker that I was using. When I disabled the ad blocker and reloaded my page, the error went away.
This happens almost always when CSS is gziped, but server returns Content-Length of not compressed resource. Seen this happen when using mod_deflate with mod_fastcgi. This is server side bug, not firefox.
Update Notice:
Firebug is out of date, instead of firebug you want to be using Firefox Developer Edition which has Firebug-type diagnostics and tools built in.
Get Firebug, or otherwise view the page source code (right click on it on Firefox and choose View Source) and click through on each .css stylesheet as referenced in the <head> section of the HTML page. One or more of these will return an Error 404 or some other error.
Each CSS sheet is in a <link> element in the Head of the HTML.
Example:
viewing the source code to this page will give a pile of code, in the <head> section is the <link>:
<!DOCTYPE html>
<html itemscope itemtype="http://schema.org/QAPage">
<head>
...
<link rel="stylesheet" type="text/css" href="//cdn.sstatic.net/Sites/stackoverflow/all.css?v=8c7d44a438e6">
...
</head>
This shows that there is a style sheet in the link element, and for your page/site if you click through all of these (there may be multiple ones) and find the one that gives you the specific error.
UPDATE:
(Update) if I look at the console of the developer's tools in Firefox, it shows all the css files and says "HTTP/1.1 200 OK" against each file.
Therefore you should be looking in each of your CSS and related documents to see which document is linking out to an unreachable resource.
Another reason for style sheet could not be loaded is "active mixed content" in combination with HTTPS. I.e. if your HTML code is loaded over HTTPS, but references a CSS that is delivered over HTTP, Firefox will block the CSS file. The blocking will cause an explanatory entry in the Web console. See https://developer.mozilla.org/en-US/docs/Web/Security/Mixed_content/How_to_fix_website_with_mixed_content for more information.
The blocking will also happen if the request for the CSS file is answered via HTTPS with a redirect and the redirect URL uses HTTP. And Google Chromium will also block active mixed content over HTTP and make an entry in its Javascript console.