Where XLSX came from
XLSX is Microsoft Excel. It belongs to the spreadsheet family, which webp.now handles by rendering the document exactly as it would print and encoding each page as an image.
The technical picture
Compression: Zipped XML or binary container. Transparency: Not applicable. Animation: Not applicable. Colour: Document colour. Typical file size relative to WebP: varies with page count and embedded media.
Where it opens
Microsoft Excel and compatible office suites. This is usually the deciding factor when choosing between XLSX and WebP — WebP renders in every browser but is uneven in desktop software, while XLSX has a very different support profile.
Converting XLSX to WebP
Each sheet is rendered as it would print and saved as a WebP image — cell formatting, borders and charts preserved as a picture that renders in any browser without exposing the underlying formulas. Formulas, cell values and chart data are gone once rasterised — the result is a picture. Keep the workbook for anything that still needs to calculate.
Converting WebP to XLSX
Convert WebP to XLSX when the destination requires it. Expect the XLSX output to be varies with page count and embedded media — that difference is the price of getting a format the destination will actually accept.