[KevinH]

All F7 does, is make sure the file is parseable, not that it is correct (it is a sanity checker). If a file is not parseable then, until it is mended, nothing can really be done with it.

Again, as an editor Sigil will allow you to type anything you want. If Preview can not parse it or sanity check (F7) fails, you typed something with incorrect syntax for xml/xhtml and it needs to be mended. You can type in links or hrefs or src's whose targets do not exist. As long as the syntax is pareseable, all is fine from the editor's perspective given the author's next step may be to add that missing target. There is no way for an editor to know.

Again, if you use the Insert Image tool, it will correctly match the actual filename preventing the problem you yourself created.

If you are worried about link, targets, etc, you can run the Report features to visually check things or even better yet run an actual validator! Epubcheck will happily point out you messed up.

Sigil is an editor, not a validator. Sigil allows validator plugins to be run inside the Sigil application.

Before considering an epub complete, I **strongly** recommend you run it against a validator to verify you have not messed anything up.