HTML compacting is one functionality that is missing in PHP. What does it do? Go to any google.com page and inspect the page source. You’d see all the unnecessary whitespace is removed. You would think it’s a small change, but look on some generated pages on other sites – especially one produced by CMSes using some designer templates – which are formatted to be human-readable and contain a bunch of comments, etc. – and you’d see that whitespace can easily double your page size.
It is not really possible, however, to make your HTML designers abandon the use of whitespace or comments. In fact, you won’t do it anyway – such code would be impossible to maintain. But an extension which can produce “compacted” HTML would be very nice. Like the tidy extension, but tidy doesn’t really know to do whitespace compacting, AFAIK. Maybe it can be modified to do that.