Add support for all Fill Effects
In Excel Desktop, open the formatting cells interface. Select the Fill tab. Click on the Fill Effects... button. Set up a fill effect for the cell, save the workbook, and open with Excel Online.
Using a horizontal gradient Color 1 -> Color 2, Excel Online renders the fill effect correctly.
Using a horizontal gradient Color 1 -> Color 2 -> Color 1, Excel Online renders only the top half of the gradient in the top half of the cell. The bottom half of the cell is white.
Using a diagonal gradient Color 1 -> Color 2, Excel Online fills the entire cell with color 2.
The other fill effects have similar oddities, some work, many don't.
See the attached images for how the same horizontal fill effect renders differently between the desktop and excel online (same issue in IE 11, FF, and Chrome)
Vertical Color 1 -> Color 2 also works
ALSO before finding this post I tried pattern fills - they don't work either.
Real world scenario - needed a status flag to highlight minor rework requirements on a project task. Yellow for in progress, Green for done, Desirable but non-Essential Rework in progress - task is complete but we need to know work is still being done. A yellow and green gradient. Now vertical actually looks better than diagonal but its not so much looks as functionality I needed and I'd put in a diagonal gradient so having to change it was infuriating.