the steps of Selection.extend() does not check whether the given offset is valid in the container

[masayuki-nakano]

The collapse() method section has the following:

The method must throw an IndexSizeError exception if offset is longer than node's length and abort these steps.

However, the extend() method section does not have the corresponding step.

I think that the check should be done before or after the 2nd step.

[masayuki-nakano]

Oh, and also these things should be checked too:

1. If node is null, this method must behave identically as removeAllRanges() and abort these steps.
2. If node is a DocumentType, throw an InvalidNodeTypeError exception and abort these steps.