Thank you for your reply, leotemp. That is a good question.
There seems to be a trend towards standards-based
development. On many fronts, including the W3C, the development
community is encouraged to build sites in which the HTML and the
CSS validate. If we trust those who are charged with the
responsibility to be informed in these matters to steer us in the
right direction, it is presumably appropriate to build
accordingly--even if the app can be "seen" without validating.
The attempt here is to work within the standards. We can't
very well complain (not that you are) that the browsers need to
recognize standards, if we in the development community choose to