In contrast to previous specs, HTML 5 specification facilitate semantic tags. Technical changes specified on that document are more to browser developer. Browsers tends to be permissive to unknown tag, it will degrade gracefully rather than showing errors.
In web developer perspective, we learn the specification and should be not force unnecessary idealism to much except on how the elements should be placed on right place, for example, we shouldn’t put form inside a form. Why? because it state on specification that the form elements can’t be recursive. That kind of idealism should be kept.
Another example, other web developers use header while you are not using it. You are using div tags, with id named header. Wrong? of course not but you’re not utilize the specification to the maximum level. Is it the content will be broken? Well, in short no.