Since converting NSP to XCI requires significant processing power and file handling (often involving large files 10GB+), a true client-side "online" converter (running in a browser) is technically impractical and rare. Most "online" sites are scams or simply host the desktop tool by tinfoil (or similar variations by hacbpc).
: Modifying or converting game files is often used for pirated content. Using converted files while connected to official Nintendo servers carries a high risk of a permanent console ban XCI vs NSP