In case it is helpful here is a link to a Unity documention page that describes how the Canvas Scaler is involved in scaling the User Interface: Unity - Manual: Designing UI for Multiple Resolutions It is titled Designing for Multiple Resolutions which may be helpful when the size of the window frame of a website is smaller or larger than the game as viewed in Unity.
I previously had trouble with Canvas Scaling and setting the Canvas Scaler to either Width or Height didn’t fix the issue so I set it to match both Width and Height by setting the Match Value to 0.5 like you can see here:
Now, I always set the Canvas Scaler to Match the Width and the Height. Then I can more easily deal with window size issues that are specific to certain Website UIs.