col value is the column WHEN the error becomes obvious to the checker.
The actual error can be somewhere before
eg
You open a <div, but it never closes. When you hit </body, all tags inside the body must have been closed: <bzzz> Error, but it does not know where it belonged
Kovid is going to add Tag Matching, but that only helps so far
Other less obvious
Open a <h3, close a </p
While you can use a <div pair for a paragraph ... using <p prevents the missing </div problem because you can not start another <p until the previous closes