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Standard Ebooks Toolset vs Sigil: catalog facts
Standard Ebooks Toolset	Sigil
Free Free and open source.	Free Free and open source under the GPL.
macOS, Linux, Command line	Windows, macOS, Linux
Lints ebook source against a detailed style guide Builds finished EPUB files	Edits EPUB 2 and EPUB 3 in a visual view or directly in the XHTML Built-in validation catches the errors that make readers reject a book
Command line only Built around the Standard Ebooks conventions	EPUB only; it does not handle MOBI, AZW3 or PDF Some XHTML and CSS knowledge is needed to fix anything non-trivial
Downloadable app	Downloadable app
Open source	Open source
License: GPL-3.0	License: GPL-3.0
No account needed	No account needed
Works offline	Works offline
Official website Full listing and sources Checked October 2, 2026	Official website Full listing and sources Checked September 21, 2026

Catalog facts only. Anything “not stated” is unconfirmed. Check full listings for details.