About this post
This is for everyone who is using the Vivaldi browser. In case you wondered why links are not focusable when you're tabbing through a website, I can let you know: You're not alone!
The solution is stated in a Vivaldi forum post:
This has to be enable in Vivaldi Settings > Webpages > Webpages Focus > (*) Focus All Controls and Links.
So for unclear reasons the Vivaldi team decided to exclude links as focus-able areas per default and each user has to change this setting on its own.
That's why I opened a feature request and a bug report (ticket: VB-73381) to change the setting's default value.
But for now you know how to fix this A11Y issue on your own in your Vivaldi browser settings.