I think one big issue is that there are OS specific APIs on Windows and Mac that allow you to only redraw the pixels/layers of the web browser window that have changed, with the unchanged portion retained in memory and recomposited.
We've seen versions of Safari and Edge that leverage those sorts of APIs to deliver better battery life on their respective platforms, but if you are writing a cross platform browser you may not be willing to do extra work for each individual platform.
Several years ago, Firefox adopted the solution Chrome used of dividing the web browser window into large-ish sections, so you could at least skip redrawing sections that had not changed, but that still leaves you doing unnecessary work, just less of it.
Prior to that, the battery drain using Firefox vs Safari on newer machines with higher resolution displays was very noticeable.
We're talking about using features exposed by the OS (the Core Animation layers API in this example) to avoid doing unnecessary work to save battery life.
Any browser could adopt each individual platform's proprietary graphics APIs, but using the same API everywhere (traditionally OpenGL) is less work.
Any browser engine is free to do the work to use per-platform APIs. If your argument is that cross-platform browser engines won't do the work to optimize with platform-specific APIs but that Safari will, WebKit is a cross-platform browser engine and so it adopting CoreAnimation on Apple platforms in addition to supporting other platforms is exactly the same amount of work that it would take for Blink or Gecko to do the same thing.
We've seen versions of Safari and Edge that leverage those sorts of APIs to deliver better battery life on their respective platforms, but if you are writing a cross platform browser you may not be willing to do extra work for each individual platform.
Several years ago, Firefox adopted the solution Chrome used of dividing the web browser window into large-ish sections, so you could at least skip redrawing sections that had not changed, but that still leaves you doing unnecessary work, just less of it.
Prior to that, the battery drain using Firefox vs Safari on newer machines with higher resolution displays was very noticeable.
https://bugzilla.mozilla.org/show_bug.cgi?id=1422090