Flagging lots of websites as "unsafe"
Vivaldi is flagging lots of websites as unsafe. Very annoying. Please fix
Those websites likely use SHA-1 security certificates which are in fact insecure. The major browser vendors have announced 2 years or so ago that they will deprecate those certificates. Most browsers have started to discontinue support for SHA-1 certificates at the beginning of this year or will do so within the next couple of weeks, if I remember correctly. So there is a good chance that those websites will fix their certificates very soon.
Some certificates may be invalid/untrusted as they use SHA1.
Google (for Chrome, Chromium and similar browsers like Vivaldi) announced end of 2014 that SSL certificates with SHA1 sign will be not truested after Jan 2017.