"is not declared for element"
styling exists within a tag that is not preceded by style=" the style "
IMHO all styling belongs in a stylesheet or the styles section in the <head>
"is not allowed for content model"
The rules of placement have bee violated
Usually
naked objects (not surrounded by valid
object type tags)
"no declaration found for element"
"element "center" is not allowed for content model"
"attribute "caption" is not declared for element ´img´"
"ID value "heading_id_13" is not unique"
There are 2 or more lines with the same id="xx" value: edit out all but the one you want any link to hit (includes the TOC).
Sigil
will duplicate
these if you split a line that has one in it.
IMHO this is a feature as the part you really want is NOT the original beginning
"attribute points to file <> which does not exist "
Guessing. Typo/incomplete filename referenced.
common :
src="../Text/#idvalue" which happens after a delete of the referenced file. there should have been a filename.(x)html before the #