This was reasonably easy in the early years of the web. But starting in 1997, the notion of a canonical browser used by everybody had been abandoned. Non-traditional computers, such as WebTV, PDAs are becoming more prevalent. Also speech based browser will happen soon. A solution for maintaining presentation encoding is to separate content and display-specific instructions.
Style sheets are a relatively new development for presentation.
Alignment - items on the page are lined up with each other. Lack of alignment is the most prevalent problem on the web. Items can be aligned left, right, centered. Choose one and use it for the entire page. Do not mix alignments.