Depends on what you mean by "better". I would rather software not handle a piece of data at all, than handle it erroneously and changing the data without me realising and thus causing all sorts of issues after.

In practice, web browsers accept the tag soup that is sometimes called html and strict xml-based formats failed.

The browser are not a database (unlike excel). Modifying data before showing it is reversible, modifying it before storing it is not.