HTML is not particularly complicated; it is essentially a simple markup language, and it is unlikely that any HTML "trick" will remain secret for long. Because HTML is simple, this would probably be the case even if the source were not visible.
Good HTML style, on the other hand, is a subtle thing and requires a high degree of consistency and editorial sense (not always displayed in this document, I'll admit). It is unlikely that anyone will succeed in stealing your "style" using the "View Source" button, although they may pick up a few tricks.