The web is complex. For the end user everything works like magic. A text of one color, another of another, a responsive image, another that remains fixed in the background... That has its work behind it, and it does not work the same in all browsers. To control things a bit, in theory, the User-Agent was invented, and Vivaldi he is playing with him because he is not treated as he thinks he deserves.
Vivaldi 6.1 introduced a novelty that we allows you to access Bing Chat (if you want…) without changing your browser. It's a small move, but I wouldn't rule out growing it in the future. As its CEO explained, the problem begins with the way your browser is treated. They would love to introduce themselves as Vivaldi, but there are web pages that don't work if you put that name in the User-Agent. Is it to annoy?
Vivaldi... it's just a "suit"
I would say not. I am a Vivaldi user, but I have also made my first steps in web design. When you go from HTML to CSS, most of the testing is done for Chrome, and then you also look at how things look in Firefox and Safari. That's because, although there are more, actually there are only three engines valid in the market: Chromium, Gecko and AppleWebKit. And as business cards, in the User-Agent it is worth putting those of Chrome, Firefox and Safari.
Vivaldi I wanted to put its name instead of Chrome's, but there are web pages that penalize it and it does not show some things. To know exactly why, you would have to see how those pages are designed: it is likely that there are CSS rules that only work if the User-Agent matches a specific string, and it is not ruled out that there is something to restrict access to some browsers. , as is the case with Vivaldi.
Not controlling certain things destroys the user experience
At first, the solution seems clear: ignore the browser. But this can have dire results.. For example, in an exercise I did, Firefox did not honor the CSS property sticky because I used it in combination with another table cell. Even worse, if you want to put a fixed background image, Apple's phone does not respect it, and on top of that it can show something blurry.
In the end, the designer wants his pages to look as good as possible on the maximum number of devices possible, and for this, he usually works first for the most used browser, Chrome, then he thinks of Safari and Firefox or vice versa. The rest don't count. So, if someone introduces himself as "Vivaldi", the website thinks it's not supported, and restricts him some things for his own good... in theory.
Different is the case of pages like Shazam's: If you do not enter with an Apple browser, it shows information on how to download the application. That is a complete restriction.
The solution is to do extra work to earn very little
Yes. The best ethical solution is to include all known browsers in the options and display the content to them in the same way that it is displayed in the browser with the shared engine. But, of course, if Firefox is already little used compared to Chrome or Safari, the latter thanks to the iPhone and iPad, and macOS in the US, take into account browsers like Vivaldi separately is to do I work too much to earn very little.
I have come to witness comments from professional design companies that have joked that if someone does not see something in Safari, buy an Android or see it from the PC. It is said as a joke, but it is a bit what they do. And as long as this continues, companies like Vivaldi Technologies don't just use a better looking User-Agent in general; now they change it depending on the service in which it is visited. If in the end there is no evil that does not come for good.