Currently following features are not supprted:
Conditional formatting
Macros and VBA code
While SpreadsheetWEB doesn't support macros, it can still execute workbooks containing macros. It will execute those workbooks as if macros were disabled.
SpreadsheetWEB has a builtin scripting support to substitute macros in Excel.See our scripting support pages Scripting Support
Third party addins
Some cell formats
Some Excel formulas
List of supported Excel formulas