These two may become an utter hell, especially when building on existing template, and upon erasing the piece of code mixed with HTML, you discover that page design is ruined. So, are there any tools to tell you, that div is unclosed and float is uncleared (and maybe some more of common HTML errors)?
Firebug is excellent and will display broken markup in the console on load. It also has excellent CSS debugging tools, such as the ability to toggle, edit live CSS(add/remove selectors and styles). It is the Swiss-Army Knife of development.