Apple is beta testing a new look for the iCloud Mail experience on the web to make it more in line with the design changes in macOS Big Sur. As a result, you can expect an overall cleaner look and feel than what's available right now. It will also allow you to compose emails in a smaller window in the middle of the screen instead of opening a new window. The redesign is expected to go live for all users in the fall.
Source: MacRumors