I know a lot of you out there are already aware of this and to you it will seem obvious, however, I've found myself giving this tip a few times over the past couple of weeks, so think it's worth quickly noting.
If you have a containing element, (div, article, section etc…), with a lot of content inside, reference which one it is at the closing tag. E.g:
<section role="main"> <!-- ALL the page's content here --> </section><!-- this is the end of section[role="main"] -->
This is particularly helpful if you have your HTML file split into partials. You may close a container in a different file to that which it starts.