[kovidgoyal]

Originally Posted by eschwartz

In most other things they are feature-comparable, so it is pretty much preference (and Sigil has a more established presence).

I really should put aside the time someday to write a comparison document, for pretty much every feature that exists in both, the calibre editor has a superior (either more powerful or more robust or both) implementation.

1) Spell check - calibre editor supports spell checking in multiple language documents and uses ICU for word iteration, which means it works with more "exotic" languages. It checks text not just between tags but also intelligently in attributes such as alt and title that contain user visible text.

2) Syntax highlighting -- the calibre editor highlights syntax errors, highlights document features, like headings, and is asynchronous, so has works much better on large documents, despite being written in an interpreted language.
- Works in CSS inside <style> tags
- Highlights links in the html and shows links that are broken. You can even Ctrl+Click on links to follow them from right in the editor code view. For example you see a link to a CSS stylesheet and want to edit it, just Ctrl+Click it in the code view.
- Autocomplete for links, closing tags and even anchors (the part after # in a link)
- Highlights matching HTML tags robustly
- Highlights invalid HTML live as you type
- Invisible characters are highlighted -- such as non-breaking spaces, special hyphens, etc.

3) Search and replace - The calibre editor has function mode, allowing you to perform very powerful transformations on your text. Sigil is missing this.

4) Live CSS - the calibre editor shows you the CSS that applies to the text you are currently editing -- it tracks your cursor, unlike the inspect element based Sigil equivalent (which the calibre editor also has).

5) The calibre editor has built in diffing which can be used both across ebooks and across checkpoints and after any complex/bulk operation. Perform a global search and replace and the calibre editor will show you exactly what changed nicely highlighted and allow you to revert all changes if you dont like them. Sigil has no diffing (at least that I could find). Nor does it have any concent of checkpoints at all, so if you perform potentially destructive/dangerous operations, you have to remember to manually save backups each and every time.

6) TOC Editor: The ToC editor allows you to choose toc locations by browsing the rendered view of your book rather than having to remember/hunt for ids. It allows you to auto-generate ToCs based on arbitrarily powerful XPath expressions.

7) The calibre editor uses an atomic write technique when saving your ebook file which means it can never corrupt your file even if you suffer a power failure in the middle of saving. And I'll just mention that Sigil for years had multiple bugs that could corrupt your document spontaneously. Those problems are mostly under control nowadays, but the calibre editor never had such problems thanks to what is, in my opinion, are much more robust internal design.

8) The calibre editor is internally designed to allow editing multiple (html based) document types, not just epub -- witness the support for editing azw3.

9) The calibre editor has a much more powerful implementation of snippets, with placeholders and convenient tabbing between them -- allowing for all sorts of advanced things -- http://manual.calibre-ebook.com/snippets.html

10) The editor has online help for html tags and css properties by simply right clicking on them.

11) The calibre editor's Check Book feature is builtin and can actually auto-fix many types of errors and also gives you detailed explanations of the meaning of errors. Not to mention that it actually checks for errors that cause real world problems instead of relying on sterile schema checks.

12) Builtin in font embedding/subsetting tools that actually understand font files in detail and parse GSUB and GPOS tables when subsetting.

13) A much more powerful plugin system which allows you to actually create plugins that extend the user interface not just work on an offline copy of the book passed to them by the main program.

14) Finally, the calibre editor does not make changes to your files just by opening them and it does not insist you follow a particular naming convention for organizing files and folders inside your epub.

Dont get me wrong, I'm glad that Sigil exists -- and I only created the calibre editor because at one point it looked like Sigil would cease to exist. Diversity and competition is good, I'm sure Sigil has good points that justify the preferences of those that prefer it over the calibre editor -- probably the biggest feature it has that the calibre editor does not is a WYSWYG mode.