VBA macros aren't supported. Last time I checked even conditional formatting wasn't supported, although I see now it was added in January 2022 [1]. Even so, as with many other features, the web UI is different and more limited. Links to documents stored locally or on an SMB share don't work. Scrolling through thousands of rows is laggy. Generally, Excel Online is useless for the complex spreadsheets used in large enterprises, which themselves are often workarounds for the limitations of the web apps the enterprise is supposed to run on.

[1]: https://techcommunity.microsoft.com/t5/excel-blog/new-condit...