Style and Structure Guidelines
clear structure for your HTML documents makes them easy to read, easy to follow, and easy to modify.

If you disappeared tomorrow and other people had to come in to work on your HTML files, could they figure out what you've been doing . . . or would it be like looking at a big messy glob of tags and text? It's easy to end up with spaghetti code. That's why one of the first steps, in learning how to create Web pages, is to learn some simple structural guidelines.