It has some cartel-like aspects but lacks others, probably because the software industry has a unique structure in which there are nearly no distribution costs.

Cambridge Dictionary: A cartel is a group of similar independent companies or countries that join together to control prices and limit competition. It involves restricting output, controlling prices, and allocating market shares.

Group of similar but independent companies (check) that join together to control prices (no, but they do join together to control the web in other ways), and limit competition (yes, by constantly adding features to HTML whilst market dumping they prevent competitors from arising). It involves restricting output (not in the literal sense, does apply if you consider the synchronized way they implement standards), controlling prices (yes, forcing them to zero instead of the natural market rate), and allocating market shares (yes, if you consider iOS browser restrictions).

I remember that apple, google, ms had some anti-poaching agreements in place, while not directly related it seems that that is also pattern of cartels to have informal agreements that hinders competition.

But they are not joining together in any way. In fact they have transactional relationship which is opposite of cartel.

They all have contracts with Google, work on the same shared specifications and in the case of most browser companies, work on the same shared codebase. That would count as a joining together for the purposes of cartel law.

That does not make them cartel. All are working for their own interests and Google is paying them. Cartels work together.

If other browser outbids Google, the members of your "cartel" will shift to them.

By that standard, isn't Linux or really any large enough open-source project a cartel?