Google's Android Messages app and the RCS standard are apparently getting a web app in the near future, but a recent commit in the Chromium Code Repository, found by XDA Developers, seems to point to Chrome OS integration that may offer some sort of extra functionality. Users will pair a compatible smartphone packing Android Messages with their Chrome OS device, such as a Chromebook or set-top box, though there may be a different procedure for eSIM-enabled Chrome OS devices with cellular connectivity, if those are indeed on the horizon. A second commit that has not yet been put into the codebase proper says that the pairing will happen within the browser.
What extra functionality Chrome OS users will get over those on Windows or Mac devices using the web client, if any, is not discussed in this commit. Since pairing takes place in the browser, integration of the browser and the Android Messages web client of some sort is a given. Chrome OS devices come in many forms, including laptops, tablets, set-top boxes and mini PC form factors, just to name a few. This means that the integration could manifest a number of ways, such as notifications that would wake a tablet or 2-in-1, or a floating or sidebar client for laptops that wouldn't disturb whatever work the user is doing. For now, of course, this is all just speculation; it does seem strange that Google would offer Chrome OS integration with a web client already on the way, but until more commits come out or something is officially announced, there's really nothing concrete as to how it will all work.
Google has been pushing hard to make RCS the new go-to standard for mobile messaging. The standard is compatible with other apps, but with Google being the main force pushing it out into the mainstream, Android Messages is the main app that's showcasing RCS. It's not a Google-owned standard, to be clear; it's a GSM standard just like the SMS protocol that it's supplanting. RCS offers support for a wide range of features, such as richer media messages, faster data transmission standards and rich messaging for businesses that can include things like boarding passes and direct links to video meetings, to name a few.