You're probably using an http://
link to a stylesheet on an https://
website.
Secure websites are not allowed to mix protocols. Everything has to be embedded from a secure server. Browsers will ignore/block HTTP resources on HTTPS pages (with varying degree of strictness).
The reason for this blocking is that insecure HTTP resources like stylesheets and scripts could still be modified by an attacker and used to spoof/hijack secure parts of the site.
If the stylesheet is served from your server, then omit protocol+host part of the URL, i.e. instead of http://example.com/style.css
use /style.css
as the URL, so it'll work on both HTTP and HTTPS. You can also use protocol-relative URLs.
If you have to have one full URL, then use https://…
URLs only.
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