First off no, I don't see why that wouldn't technically be valid HTML5 though there have been some very odd things in the HTML5 spec so always best to ask.
Secondly what you'll want to do is standardize your CSS. Are you going to randomly style elements across a site without consistency or are you going to have an idea of how you previously approached it and implement a more uniform and easier to work with solution? Quick and easy fixes quickly accrue long term damage to the time you will end up spending more time on maintenance than adding new content and features.
You should use those practices in
all your work. Working with other people and their code? Get them in to good practices such as never using padding (except in the dumb collapsed margin issue for paragraphs). When you and the people around you start using good coding practices you stop getting stuck on prolonged maintenance and you get more done in less time.