Accessible content provides Quail with a list of the css files which would be included (from accessible_content_get_css_files). This list can be incorrect.
Themes can remove a css file which has been added by a module, by adding an entry for the css file in the theme's info file (and not having the css file in the theme).
This would normally remove OG's css (assuming sites/all/themes/foo/og.css doesn't exist).
stylesheets[all][] = og.css
At the moment, if this theme feature is used, accessible content will include http://example.com/sites/all/themes/foo/og.css in the list of css files to pass to Quail. Quail then requests this file with curl (getting 404'd), which adds significant performance overhead.
The attached patch mimics the approach used in the core theme engine, which checks that the css file exists before adding it to the $vars['styles'].
Needs review
1.1
Code
Not all content is available!
It's likely this issue predates Contrib.social: some issue and comment data are missing.
I suspect it was to be set as "Reviewed & tested by the community" but the user misclicked. I set it as Needs Review for now and anyone is free to check the patch and either review it or create a MR based on it with any possible corrections.