The release notes for Vivaldi 1.1.453.59 aren’t exactly overflowing with information. The current version is only referenced in the page URL, while previous versions are listed below, in a series of rather confusingly-titled and somewhat redundant sections that all look like this:

Changelog since 1.1, the fourth release.

Parsing what information is available, I concluded that this version was released to fix one security vulnerability: [Security] Address bar spoofing using HTTP status code 204/205.

It’s a security update, so if Vivaldi is your browser of choice, you should navigate to Help > Check for Updates... on the Vivaldi menu.