Daniel Osei

Principal Engineer

Daniel builds the parsing and conversion engines behind ExcelToolkit Pro. He has spent more time inside the OOXML specification than is healthy and enjoys explaining why your file is 40 MB when it only has 3,000 rows.

OOXML
File formats
Browser performance
WebAssembly

github.com

Articles by Daniel

Security & privacyJune 2, 20252 min read

What your spreadsheet reveals about you when you email it

Author names, internal folder paths, hidden sheets, tracked revisions and comment authors all travel with the file. Most people never check.

DODaniel OseiPrincipal Engineer

PerformanceApril 22, 20252 min read

Your 3,000-row workbook is 40 MB. Here is where the bytes went.

File size rarely correlates with data volume. It correlates with style records, phantom used ranges and pivot caches nobody knew existed.

DODaniel OseiPrincipal Engineer