After making it available for the Android app, Google is rolling out support for emoji reactions to Messages on the web. The feature seems to be rolling out to all users via a server-side update.
Messages for web RSS
Google seems to have started transitioning Messages for web to its new domain, messages.google.com, apparently as part of the messaging service's departure from its Android.com domain.
Google announced today that it's beginning its rollout of Messages for web, allowing users to send SMS messages from the browser, meaning that you can access your phone from any device.