The term "evergreen browser" refers to browsers that are automatically upgraded to future versions, rather than being updated by distribution of new versions from the manufacturer, as was the case with older browsers. The term is a reflection on how the design and delivery of browsers have changed quickly over the last few years, as technology advances in general and various new players threaten Microsoft's dominance as in the early days of MS Internet Explorer.
Techopedia explains Evergreen Browser
Google Chrome is an example of an evergreen browser. Google Chrome is frequently updated without user intervention. Now, Mozilla Firefox and Microsoft Explorer are both moving toward an evergreen-browser approach. It is projected that, soon, all browsers will be self-updating.
The issue of evergreen browsers raises some questions for Web developers. Although in some cases it can be difficult to deal with quick browser changes, the idea is that, with automated operations with evergreen browsers of the future, Web developers will have to worry a lot less about what version of a browser they are dealing with. The futuristic model for browser design and successive versions assumes that more compatibility will be built into these models so that Web developers do not have to keep constantly adapting to new version changes.
The term "evergreen browser" comes from the more general term "evergreen," which is used to describe various technologies such as websites that refresh constantly to remain fresh and useful. There is also the associated term "evergreen" used in journalism, which refers to digital or print content that is always applicable and always relevant, regardless of chronology. This relates back to the evergreen browser, which is expected to be always applicable and always relevant to the structuring of outside applications. In other words, the same technologies would apply in future versions of the browser.