Hi Emanuele,
At this stage, the Spreadsheet component does not offer support for HTML. There is already a Feature Request on this functionality available here:
https://feedback.telerik.com/kendo-jquery-ui/1359632-spreadsheet-render-cell-using-custom-html
I have related this thread to the one above and your vote will be transferred to the Feature Request thread linked above.
Regards,
Aleksandar
Progress Telerik