[Tex2002ans]

Epubcheck is the only one that matters, because it's:

• The only checker actual stores (B&N, Amazon, Kobo, etc.) use.
• It's following the actual EPUB specs.
• It's consistently updated/maintained.

FlightCrew was just from the early days of Sigil to help:

• validate + catch some basic errors in one-button press.
• jump to the exact line # by double-clicking on the error.

Now that you have Doitsu's fantastic plugin, that lets you easily run epubcheck just like you used to run FlightCrew.

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So:

• If epubcheck complains, definitely take a look + fix the problem.
• If FlightCrew complains, take a closer look. It may or may not be an actual issue.

Very simple (and correct) books, both of them will show 0 errors.

If you are using much newer (or more complicated code), FlightCrew might complain, because that type of stuff may not have existed (or been expected) way back then.

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Side Note: On marking up your book's language with HTML lang + xml:lang... you can follow all the same advice given previously in:

• 2019: "Trying to simplify my work flow (confused about the heading/language again)"
• 2018: "Two Questions"

The only thing I'd do differently now compared to back then is... capitalizing that "en-us" -> "en-US".